Update ISSUE_TEMPLATE to call out FCM issues

Author: samtstern

ISSUE_TEMPLATE.md

@@ -2,6 +2,7 @@
 ### Step 1: Are you in the right place?
 
   * For issues or feature requests related to the code **in this repository** file a Github issue.
+    * If you are filing an issue about **FCM in the background** make sure to read [#4](https://github.com/firebase/quickstart-android/issues/4) and [#89](https://github.com/firebase/quickstart-android/issues/89) first!
   * For general technical questions, post a question on [StackOverflow](http://stackoverflow.com/) tagged appropriately.
   * For general Firebase discussion, use the [firebase-talk google group](https://groups.google.com/forum/#!forum/firebase-talk)
   * For help troubleshooting your application that does not fall under one of the above categories, reach out to the [personalized Firebase support channel](https://firebase.google.com/support/contact/troubleshooting/)