Update Pull Request Contributing guidelines.

Pull requests are coming in with a wide range of
quality. There's no baseline of expectation being set
in current documentation. This is a first step
towards providing it for both external contributors
as well as amongst members of the core Pony team.

CONTRIBUTING.md

@@ -31,8 +31,28 @@ examples.
 
 Pull request
 ------------
-Provide the same information as you would for a [feature request](#feature-request). Also briefly
-describe what the implementation might look like.
+Provide the same information as you would for a [feature request](#feature-request).
+
+Before issuing a pull request we ask that you squash all your commits into a 
+single logical commit. While your PR is in review, we may ask for additional
+changes, please do not squash those commits while the review is underway. Once
+everything is good, we'll then ask you to further squash those commits before
+merging. We ask that you not squash while a review is underway as it can make it
+hard to follow what is going on. Additionally, we ask that you:
+
+* [Write a good commit message](http://chris.beams.io/posts/git-commit/)
+* Issue 1 Pull Request per feature. Don't lump unrelated changes together.
+
+If you aren't sure how to squash multiple commits into one, Steve Klabnik wrote
+[a handy guide](http://blog.steveklabnik.com/posts/2012-11-08-how-to-squash-commits-in-a-github-pull-request)
+that you can refer to. 
+
+We keep a [CHANGELOG](CHANGELOG.md) of all software changes with behavioural 
+effects in ponyc. If your PR includes such changes (rather than say a
+documentation update), please make sure that as part of PR, you have also
+updated the CHANGELOG.
+
+
 
 [complete-issue-list]: //github.com/ponylang/ponyc/search?q=&type=Issues&utf8=%E2%9C%93
 [new-issue]: //github.com/ponylang/ponyc/issues/new