Update faqs.md

Added some expanded descriptions of common problems encountered, as well as how to use the googleApiAvailability method to run availability checks.

docs/faqs.md

@@ -3,7 +3,7 @@
 ### Comparison to Firestack
 
 Firestack was a great start to integrating Firebase and React Native, however has underlying issues which needed to be fixed.
-A V3 fork of Firestack was created to help address issues such as lack of standardisation with the Firebase Web SDK, 
+A V3 fork of Firestack was created to help address issues such as lack of standardisation with the Firebase Web SDK,
 and missing core features (crash reporting, transactions etc). The volume of pull requests with fixes/features soon became
 too large to manage on the existing repository, whilst trying to maintain backwards compatibility.
 
@@ -42,12 +42,74 @@ export function onAuthStateChanged() {
 }
 ```
 
-### [Android] Google Play Services is required to run this application but no valid installation was found
+### [Android] Google Play Services related issues
 
-The emulator/device you're using does not have the Play Services SDK installed. 
+The firebase SDK requires a certain version of Google Play Services installed on Android in order to function properly.
 
-- Emulator: Open SDK manager, under 'SDK Tools' ensure "Google Play services, rev X" is installed. Once installed, 
+If the version of Google Play Services installed on your device is incorrect or non existent, React Native Firebase will throw a red box error, and your app will possibly crash as well. The red box error will have a numerical code associated with it. These codes can be found here:
+
+ https://developers.google.com/android/reference/com/google/android/gms/common/ConnectionResult#SERVICE_VERSION_UPDATE_REQUIRED
+
+Here is a quick guide to some of the most common errors encountered:
+
+code 2 -  Google Play Services is required to run this application but no valid installation was found:
+
+The emulator/device you're using does not have the Play Services SDK installed.
+
+- Emulator: Open SDK manager, under 'SDK Tools' ensure "Google Play services, rev X" is installed. Once installed,
 create a new emulator image. When selecting your system image, ensure the target comes "with Google APIs".
 - Device: Play Services needs to be downloaded from the Google Play Store.
 
+code 9 - The version of the Google Play services installed on this device is not authentic:
 
+This error applies to modified or 'shimmed' versions of Google Play Services which you might be using in a third
+party emulator such as GenyMotion.
+
+Using this kind of workaround with Google Play Services can be problematic, so we
+recommend using the native Android Studio emulators to reduce the chance of these complications.
+
+### [Android] Checking for Google Play Services availability with React Native Firebase
+
+React Native Firebase actually has a useful helper method for checking G.P.S availability:
+
+```javascript
+const availability = firebase.googleApiAvailability;
+```
+
+The availability object would then have the following properties that you can run checks against:
+
+```javascript
+isAvailable: boolean
+```
+
+and if not available (isAvailable === false):
+
+```javascript
+isUserResolvableError: boolean
+```
+
+This variable indicates whether or not the end user can fix the issue, for example by downloading the required version of Google Play Services. In a case such as a GenyMotion emulator, this would return false for missing G.P.S, as the end user can't add the package directly.
+
+```javascript
+error: string
+```
+This error will match the messages and error codes mentioned above, and can be found here:
+
+https://developers.google.com/android/reference/com/google/android/gms/common/ConnectionResult#SERVICE_VERSION_UPDATE_REQUIRED
+
+
+### [Android] Duplicate Dex Files error (build time error)
+
+A common build time error when using libraries that require google play services is of the form:
+'Failed on android with com.android.dex.DexException: Multiple dex files... '
+
+This error (https://github.com/invertase/react-native-firebase/issues/48) occurs because different versions of google play services or google play services modules are being required by different libraries.
+
+The process to fix this is fairly manual and depends on your specific combination of external libraries. Essentially what's required is to check the app level build.gradle file of each external library and establish which ones have a Google Play Services dependency.
+
+You then need to find the lowest common version of each G.P.S module dependency, require that in the app level build.gradle file of your own project, and exclude it from being required by the modules themselves. This will force the use of a consistent version of the G.P.S module.
+
+It's not a good idea to modify the version within the library's build.gradle, as this will be overwritten when you update the library, which will lead to the build breaking again.
+
+A good break down of this process can be found here:
+https://medium.com/@suchydan/how-to-solve-google-play-services-version-collision-in-gradle-dependencies-ef086ae5c75f