Added a 'CONTRIBUTING.md' file (#2523)

* Added a 'CONTRIBUTING.md' file

* Update CONTRIBUTING.md

Added link to blog post based on euhmeuh's suggestion

Co-Authored-By: dstorrs <david.storrs@gmail.com>

* propose shorter guide

* add link "About Pull Requests"

Add a link to a GitHub guide on pull requests (thanks to @spdegabrielle)

Remove some text & links to make the un-rendered file easier to read. The missing links are easy to find from the remaining ones.

CONTRIBUTING.md

@@ -0,0 +1,24 @@
+# Contributing to Racket
+
+Thank you for your interest in contributing to Racket!
+
+Racket lives on GitHub in multiple repositories: <https://github.com/racket>
+
+To contribute, visit a repository and open a pull request.
+
+
+#### More information:
+
+- GitHub: [About Pull Requests](https://help.github.com/en/articles/about-pull-requests)
+- Racket Wiki: [Ways to contribute to Racket](https://github.com/racket/racket/wiki/Ways-to-contribute-to-Racket)
+- Racket Blog tutorials for contributors:
+  + [2017-09-27](https://blog.racket-lang.org/2017/09/tutorial-contributing-to-racket.html)
+  + [2012-11-25](https://blog.racket-lang.org/2012/11/tutorial-contributing-to-racket.html)
+
+
+#### Other resources
+
+- mailing lists: <https://lists.racket-lang.org>
+- Slack channel: <https://racket.slack.com>
+  + Slack invite link: <http://racket-slack.herokuapp.com>
+- package index: <https://pkgs.racket-lang.org>