Add GitLab issue template

Add recommendations for the most common mistakes when reporting an
issue.

.gitlab/issue_templates/Default.md

@@ -0,0 +1,15 @@
+<!--
+
+Please read the following before submitting:
+
+- If you are a user of a wlroots-based compositor, please report first your
+  issue to your compositor's issue tracker. The issue you are experiencing
+  might be a compositor bug rather than a wlroots bug.
+- Do not open issues related to proprietary drivers.
+- Do not use this issue tracker for questions. Instead, use the IRC channel
+  (#wlroots on Libera Chat).
+- Please attach full debug logs of your compositor (redact information you
+  might consider sensitive). For crashes, please attach a full stack trace with
+  debug symbols.
+
+-->