Update Installing docs (#3658)

- Added missing step: Ignoring the build variants that we don't use or building the app module directly so it picks up the correct build variant
- Converged Step 2 and 3 into one step since they both required tweaking according to the React-Native version a project uses.

docs/docs/Installing.md

 include ':react-native-navigation'
 project(':react-native-navigation').projectDir = new File(rootProject.projectDir, '../node_modules/react-native-navigation/lib/android/app/')
 ```
+## 2. Updating Gradle build files.
 
-### 2. Update `android/build.gradle`:
+> react-native-navigation supports multiple React Native versions. Target the React Native version required by your project by specifying the RNN build flavor you require.
+><br><br>Available options:
+>
+>* `reactNative51`: Support for React Native 0.51-0.54
+>* `reactNative55`: Support for React Native 0.55
+>* `reactNative56`: Support for React Native 0.56 and above
+>
+><br>For example: To target React Native 0.56
+> <br><br>Replace the following line in step 2.2:<br>
+>`missingDimensionStrategy "RNN.reactNativeVersion", "reactNative51"`
+><br>with:<br>
+>`missingDimensionStrategy "RNN.reactNativeVersion", "reactNative56"`
+><br><br>Make a choice in step 2.3 to alter your Android run-command (**prefered**) or ignore the unused buildvariants in your root build.gradle file.
+
+### 2.1 Update `android/build.gradle`:
 
 ```diff
 buildscript {
 +		maven { url 'https://jitpack.io' }
 	}
 }
+
+
 ```
 
-### 3. Update project dependencies in `android/app/build.gradle`.
+### 2.2 Update project dependencies in `android/app/build.gradle`.
 
 ```groovy
 android {
 	defaultConfig {
 		minSdkVersion 19
 		targetSdkVersion 25
-		missingDimensionStrategy "RNN.reactNativeVersion", "reactNative51" // <- See note below for further instruction regarding compatibility with other React Native versions
+		missingDimensionStrategy "RNN.reactNativeVersion", "reactNative51" // <- See note above for further instruction regarding compatibility with other React Native versions
 	...
 	}
 
 	implementation project(':react-native-navigation')
 }
 ```
-> react-native-navigation supports multiple React Native versions. Target the React Native version required by your project by specifying the RNN build flavor you require.
-><br><br>Available options:
->
->* `reactNative51`: Support for React Native 0.51-0.54
->* `reactNative55`: Support for React Native 0.55
->* `reactNative56`: Support for React Native 0.56 and above
->
-><br>For example, To target React Native 0.55, replace the following line:<br>
->`missingDimensionStrategy "RNN.reactNativeVersion", "reactNative51"`
-><br>with:<br>
->`missingDimensionStrategy "RNN.reactNativeVersion", "reactNative55"`
 
-### 4. Make sure you're using the new gradle plugin, edit `android/gradle/wrapper/gradle-wrapper.properties`
+### 2.3 Compiling the correct build flavor
+Now that you've specified the RNN flavor you need to compile acording to the installed React Native version, you need to ignore the other flavors RNN provides.
+
+1. **The preferable option**: Use the configuration specified in the `app` module. The RNN flavor you would like to build is specified in `app/build.gradle` - therefore in order to compile only that flavor, instead of building your entire project using `./gradlew assembleDebug`, you should instruct gradle to build the app module: `./gradlew app:asembleDebug`. The easiest way is to add a package.json command to build and install your debug Android APK since the RN-CLI run-android command will no longer be of use.
+```
+"scripts": {
+  "run-android": "cd ./android && ./gradlew app:assembleDebug && ./gradlew installDebug"
+}
+```
+
+
+2. Alternatively, explicitly ignore unwanted flavors in your project's root `build.gradle`. (Note: As more build variants come available in the future, you will need to adjust this list. While this option lets you keep the CLI command ```react-native run-android``` it comes at a cost of future upkeep.)
+```diff
++subprojects { subproject ->
++    afterEvaluate {
++        if ((subproject.plugins.hasPlugin('android') || subproject.plugins.hasPlugin('android-library'))) {
++            android {
++                variantFilter { variant ->
++                    def names = variant.flavors*.name
++                    if (names.contains("reactNative51") || names.contains("reactNative55")) { // <- See note above for further instruction regarding compatibility with other React Native versions
++                        // Gradle ignores any variants that satisfy the conditions above.
++                        setIgnore(true)
++                    }
++                }
++            }
++        }
++    }
++}
+```
+
+### 3. Make sure you're using the new gradle plugin, edit `android/gradle/wrapper/gradle-wrapper.properties`
 
 ```diff
 distributionBase=GRADLE_USER_HOME
 -distributionUrl=https\://services.gradle.org/distributions/gradle-2.14.1-all.zip
 ```
 
-### 5. Update `gradle.properties` and disable incremental resource processing
+### 4. Update `gradle.properties` and disable incremental resource processing
 
 ```diff
 +# Disable incremental resource processing as it broke release build
 +android.enableAapt2=false
 ```
 
-### 6. In `MainActivity.java` it should extend `com.reactnativenavigation.NavigationActivity` instead of `ReactActivity`.
+### 5. Update `MainActivity.java`
+
+`MainActivity.java` should extend `com.reactnativenavigation.NavigationActivity` instead of `ReactActivity`.
 
 This file can be located in `android/app/src/main/java/com/yourproject/`.
 
 
 If you have any **react-native** related methods, you can safely delete them.
 
-### 7. In `MainApplication.java`, add the following
+### 6. In `MainApplication.java`, add the following
 	
 ```java
 import com.reactnativenavigation.NavigationApplication;
 ```
 Make sure that `isDebug` method is implemented.
 
-### 8. Update `AndroidManifest.xml` and set `application` **android:name** value to `.MainApplication`
+### 7. Update `AndroidManifest.xml` and set `application` **android:name** value to `.MainApplication`
 
 ```xml
 <application
     ...
 />
 ```
-### 9. Force the same support library version across all dependencies
+### 8. Force the same support library version across all dependencies
 
 Some of your dependencies might require a different version of one of Google's support library packages. This results in compilation errors similar to this:
 
     implementation 'com.android.support:appcompat-v7:25.4.0'
 }
 
-```
+```