Further improvements to CONTRIBUTING.md.

Author: jgm

CONTRIBUTING.md

@@ -1,9 +1,25 @@
 Contributing
 ============
 
-1.  Ensure that all tests pass (`make test`).
-2.  Ensure that performance has not been affected (`make bench`).
-3.  Use `make lint` to check for problems.
-4.  Changes to `dist` should not be committed.  (We will regenerate
-    `dist/commonmark.js` before a release.)
-5.  Follow the style of the existing code.
+Submitting bug reports
+----------------------
+
+There is a [forum for discussing CommonMark](http://talk.commonmark.org);
+use it for questions and discussions that might be open-ended. Use the
+[github issue tracker](http://github.com/jgm/commonmark.js/issues)
+only for simple, clear, actionable issues.
+
+Submitting pull requests
+------------------------
+
+1. A good pull request makes one logical change and does not mix
+   several independent changes.
+2. If you have several commits that successively refine a single
+   logical change, rebase them into a single clean commit.
+3. Ensure that all tests pass (`make test`).
+4. Use `make lint` to check for problems.
+5. Ensure that performance has not been affected (`make bench` before
+   and after the change).
+6. Changes to `dist` should not be committed.  (We will regenerate
+   `dist/commonmark.js` before a release.)
+7. Follow the style of the existing code.