Edited how to return additional packages (#1161)

Docs weren't very clear on how exactly to return additional packages, i.e., as a <ReactPackages> list [Line 65]. 
Also changed MyApplication to MainApplication on line 54 (if that is a default and hasn't been changed by the developer).

docs/installation-android.md

 	```java
 	import com.reactnativenavigation.NavigationApplication;
 	
-	public class MyApplication extends NavigationApplication {
+	public class MainApplication extends NavigationApplication {
  
     	@Override
 		public boolean isDebug() {
 
 	    @Override
 	    public List<ReactPackage> createAdditionalReactPackages() {
-		    // Add the packages you require here.
-			// No need to add RnnPackage and MainReactPackage
+		// Add the packages you require here like so... (Tested on RN 0.42)
+		return Arrays.<ReactPackage>asList(
+           	    new <InsertPackageName()>;
+       		);
+		// No need to add RnnPackage and MainReactPackage
+		// Simply return null if you do not have additional packages    
 	        return null;
 	    }
 	}