Add a template for issues

Summary:
This template reminds the users to use issues only for bug reports. The template is written according to the github guidelines at https://help.github.com/articles/creating-an-issue-template-for-your-repository/
Closes https://github.com/facebook/rocksdb/pull/2948

Differential Revision: D5943558

Pulled By: maysamyabandeh

fbshipit-source-id: c83b5d211ea8e334107141967689b2f0c453bbc9

issue_template.md

+> Note: Please use Issues only for bug reports. For questions, discussions, feature requests, etc. post to dev group: https://www.facebook.com/groups/rocksdb.dev
+
+### Expected behavior
+
+### Actual behavior
+
+### Steps to reproduce the behavior