Add contributing guidelines

Regarding issues/PRs relationship.

CONTRIBUTING.md

@@ -265,6 +265,19 @@ severely complicates Git history. It is intended for special circumstances, as w
 the PR branch cannot or should not be modified. If you have any questions about it,
 please ask us.
 
+### Pull Requests & Issues
+
+A pull request /fixes/ a problem that is /described/ in an issue. Make sure
+to file an issue before opening a pull request, bar from trivial PRs. In the
+issue you can illustrate your proposed design, UX considerations, tradeoffs
+etc. and work them out with other contributors. The PR is for implementation.
+
+If a PR becomes out of sync with its issue (telltale signs: the PR grew in
+scope or in the PR there are lengthy discussions about things that are /not/
+implementation choices), go back to the old issue, update it, and continue
+the conversation there (or even open a new issue if appropriate).
+
+
 Changelog
 ---------