I started editing this just to fix the link to the "Pull Requests" doc (it moved from README.md to CONTRIBUTING.md), but then I saw a few other places in need of service.
* use Markdown links for readability
* add link to pull request size limit wiki page re: code dumps
* remove link to PR 1284, it doesn't add anything here and takes the reader out of context
* add link to mailing list
* add link to testing docs