issue_templates: add some basic rules on representing postmarketOS

.gitlab/issue_templates/Default.md

@@ -52,3 +52,9 @@ If the talk was accepted and there's a public link to it, you can include that h
 <!--
 If there will be a pmOS table, briefly describe it. Requests for help, etc can be made in the issue comments.
 -->
+
+- [ ] I have read the [Code of Conduct](https://postmarketos.org/coc/) (CoC)
+- [ ] I am aware that the CoC applies to me whenever I publicly represent
+  postmarketOS, without prejudice of other CoC being applicable for the event.
+- [ ] I will be respectful towards other projects, and value collaboration over
+  competition within the Linux space.