Add debugging guide to docs (#94)

docs/Debugging.md

+# React Native WebView Debugging Guide
+
+Here are some helpful React Native WebView debugging tips.
+
+## Debugging WebView Contents
+
+### iOS & Safari
+
+It's possible to debug WebView contents in the iOS simulator or on a device using Safari Developer Toolkit.
+
+#### Steps:
+
+1. Open Safari Preferences -> "Advanced" tab -> enable checkbox "Show Develop menu in menu bar"
+2. Start app with React Native WebView in iOS simulator or iOS device
+3. Safari -> Develop -> [device name] -> [app name] -> [url - title]
+4. You can now debug the WebView contents just as you would on the web
+
+##### Note:
+
+When debugging on device you must enable Web Inspector in your device settings:
+
+Settings -> Safari -> Advanced -> Web Inspector
+
+### Android & Chrome
+
+It's possible to debug WebView contents in the Android emulator or on a device using Chrome DevTools.
+
+1. You will need to make the following change to `MainActivity.java` to enabled web contents debugging:
+```java
+  import android.webkit.WebView;
+
+  @Override
+  public void onCreate() {
+    super.onCreate();
+	  ...
+    WebView.setWebContentsDebuggingEnabled(true);
+  }
+```
+2. Start app with React Native WebView in Android emulator or Android device
+3. Chrome -> DevTools -> Menu (3 dots) -> More tools -> Remote devices
+4. Select your device on the left and select "Inspect" on the WebView contents you'd like to inspect
+5. You can now debug the WebView contents just as you would on the web
+
+##### Note:
+
+When debugging on device you must enable USB debugging in your device settings:
+
+Settings -> System -> About Phone -> Developer options -> enable USB debugging