Add issue template

Author: chkuang-g

.github/ISSUE_TEMPLATE/firebase-unity-sdk-issue.md

@@ -0,0 +1,23 @@
+---
+name: Firebase C++ SDK issue
+about: Please use this template to report issues with the Firebase C++ SDK.
+title: ''
+labels: new
+assignees: ''
+
+---
+
+### Please fill in the following fields:
+Firebase C++ SDK version: 
+Firebase plugins in use (Auth, Database, etc.):
+Additional SDKs you are using (Facebook, AdMob, etc.): 
+Platform you are using the C++ SDK on (Mac, Windows, or Linux): 
+Platform you are targeting (iOS, Android, and/or desktop): 
+
+### Please describe the issue here:
+(Please list the full steps to reproduce the issue. Include device logs, Unity logs, and stack traces if available.)
+
+### Please answer the following, if applicable:
+Have you been able to reproduce this issue with just the [Firebase C++ quickstarts](https://github.com/firebase/quickstart-cpp) ?
+
+What's the issue repro rate? (eg 100%, 1/5 etc)