Add Contributing Guide!

CONTRIBUTING.md

+
+Contributing Guide
+==================
+
+Hello, and thank you so much for taking your time to contribute in any way to
+htop! There are many ways to contribute, and I'll try to list them below. The
+support from the free software community has been amazing over the years and
+it is the number one thing that keeps me going, maintaining and improving
+something that started as a tiny pet project back in 2004 and that nowadays is
+a piece of software used all over the world, in both reality [and
+fiction!](http://hisham.hm/htop/index.php?page=sightings). Cheers!
+
+-- Hisham Muhammad
+
+Bug Reports
+-----------
+
+Bug reports should be posted in the [Github issue
+tracker](http://github.com/hishamhm/htop/issues). (I reply to them all, but I
+usually do it in batches! :) ) Bug reports are extremely important since it's
+impossible for me to test htop in every possible system, distribution and
+scenario. Your feedback is what keeps the tool stable and always improving!
+Thank you!
+
+Pull Requests
+-------------
+
+Code contributions are most welcome! Just [fork the
+repo](http://github.com/hishamhm/htop) and send a [pull
+request](https://github.com/hishamhm/htop/pulls). Help is especially
+appreciated for support of platforms other than Linux. If proposing new
+features, please be mindful that htop is a system tool that needs to keep a
+small footprint and perform well on systems under stress -- so unfortunately I
+can't accept every new feature proposed, as I need to keep the tool slim and
+maintainable. Great ideas backed by a PR are always carefully considered for
+inclusion, though! Also, PRs containing bug fixes and portability tweaks are a
+no-brainer, please send those in!
+
+Feature Requests
+----------------
+
+Back when htop was hosted in SourceForge, there used to be separate Bug
+Tracker and Feature Request pages. These go all lumped together under "Issues"
+in Github, which is a bit confusing. For this reason, I close Feature Requests
+and file them with the [`feature
+request`](https://github.com/hishamhm/htop/issues?utf8=%E2%9C%93&q=is%3Aissue+label%3A%22feature+request%22+)
+label, where they remain accessible, but not mixed with actual bug reports.
+This doesn't mean I'm dismissing or ignoring feature requests right away! It's
+just an organizational issue (with Github, really!).
+
+Donations
+---------
+
+If you like htop, feel free to [buy the author a
+beer](http://hisham.hm/htop/index.php?page=donate). :-) 
+