Add GitHub issue template to redirect users to GitLab

Author: Mordil

.github/issue_template.md

@@ -0,0 +1,5 @@
+## :warning: The GitHub repository is READ ONLY! :warning:
+
+**Please file an issue against the [source GitLab project](https://gitlab.com/Mordil/RediStack) instead.**
+
+**Issues created on the GitHub mirror will immediately be closed and not copied to the GitLab repo.**