use markdown for readme and contributing guidelines

- https://github.com/github/markup
- https://github.com/blog/1184-contributing-guidelines

CONTRIBUTING.md

@@ -0,0 +1,19 @@
+# OpenRA Contributing Guidelines
+
+## Bug reports
+
+* Have you read the [FAQ](https://github.com/OpenRA/OpenRA/wiki/FAQ)?
+* Add the appropriate log files on crashes.
+* Please be specific on how to reproduce the problem.
+
+## Patches
+
+* [Coding standard](https://github.com/OpenRA/OpenRA/wiki/Coding-Standard)
+* [Branches and Releases](https://github.com/OpenRA/OpenRA/wiki/Branches-and-Releases)
+* [Licensing](http://www.gnu.org/licenses/quick-guide-gplv3.html)
+
+Please `git rebase` to the latest revision of the bleed branch.
+
+Don't forget to add youself to [AUTHORS](https://github.com/OpenRA/OpenRA/blob/bleed/AUTHORS).
+
+While your pull-request is in review it will be helpful if you join [IRC](irc://chat.freenode.net/openra) to discuss the changes.