Clarify SDK compatibility

Fixes #218

Author: samtstern

README.md

@@ -40,6 +40,32 @@ dependencies {
 
 After the project is synchronized, we're ready to start using Firebase functionality in our app.
 
+### Compatibility with Firebase / Google Play Services Libraries
+
+FirebaseUI libraries have the following transitive dependencies on the Firebase SDK:
+```
+firebase-ui-auth
+|--- com.google.firebase:firebase-auth
+|--- com.google.android.gms:play-services-auth
+
+firebase-ui-database
+|--- com.google.firebase:firebase-database
+```
+
+Each version of FirebaseUI has dependency on a fixed version of these libraries, defined as the variable `firebase_version`
+in `common/constants.gradle`.  If you are using any dependencies in your app of the form 
+`compile 'com.google.firebase:firebase-*:x.y.z'` or `compile 'com.google.android.gms:play-services-*:x.y.z'`
+you need to make sure that you use the same version that your chosen version of FirebaseUI requires.
+
+For convenience, here are some examples:
+
+| FirebaseUI Version | Firebase/Play Services Version |
+|--------------------|--------------------------------|
+| 0.4.3              | 9.2.1                          |
+| 0.4.2              | 9.2.0                          |
+| 0.4.1              | 9.0.2                          |
+| 0.4.0              | 9.0.0                          |
+
 ## Usage
 
   * [firebase-ui-database](database/README.md)