Add issue template

Author: tulir

.github/ISSUE_TEMPLATE/issue.md

@@ -0,0 +1,21 @@
+---
+name: Issue
+about: Submit a bug report or feature request related to bbctl.
+
+---
+
+<!--
+Do not submit anything unrelated to bbctl here.
+
+For issues with self-hosting bridges with a self-hosted Matrix server (outside Beeper),
+refer to the official bridge repositories under github.com/mautrix.
+
+For issues with bridges built into Beeper, as well as Beeper client issues
+unrelated to bridges, use the "Report a problem" button in the clients.
+
+For any other Beeper-related questions, email [email protected]
+or ask in the Beeper Community room.
+
+Also, for setup issues related to bbctl, it's often quicker to
+ask in #self-hosting:beeper.com than creating an issue.
+-->