Add issue template to redirect people to appropriate channels

issue_template.md

@@ -0,0 +1,12 @@
+Welcome to the Firebase iOS Quickstarts repository!
+
+This issue tracker should be used for sample-related bugs and feature
+requests only.
+
+If you're filing a bug or feature request, please check if it already
+exists at the
+[Firebase iOS SDK repository](https://github.com/firebase/firebase-ios-sdk/issues)
+and, if it doesn't, file it there instead of on this repository's issue tracker.
+
+Please file other general programming inquiries at
+[Stack Overflow](https://http://stackoverflow.com/questions/ask).