Add Google Group to Issue Template

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/6339

Differential Revision: D19608457

fbshipit-source-id: 2adea28b1bd20b85ccafca1aa567030115220ea6

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