Add contribution guidelines

CONTRIBUTING.md

 Contributions and bug reports are welcome!
 
-Please remember to add changelog entries.
+## Contribution guidelines
+
+  * **Size of changes:** If the change is minor, feel free to just send a pull request.  Otherwise, please discuss on the issue tracker first.
+
+  * **Continuous integration (CI):** Please make sure all AppVeyor and Travis CI tests pass on your pull request.
+
+  * **New tests:**
+
+      * When implementing a new feature, please include tests for it.  (Use `tools/testctl` to add a new test module.)
+      * When fixing a bug, please include regression tests if possible.
+
+  * **Changelog:** Please update the `changelog.md` for any non-trivial change.