Add contributors guide.

This will appear when opening a new bug on GitHub.

.github/CONTRIBUTING.md

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+Thanks for contributing to Chocolate Doom! Whatever your contribution,
+whether it's code or just a bug report, it's greatly appreciated.
+
+### Reporting bugs
+
+Before reporting a bug, it's worth checking if this really is a bug.
+Chocolate Doom's mission is to reproduce the Vanilla (DOS) versions of
+the Doom engine games, bugs and all. Check out the
+[NOT-BUGS](../NOT-BUGS) file for a list of common issues which aren't
+really bugs at all. You might also try searching [the GitHub issues
+list](https://github.com/chocolate-doom/chocolate-doom/issues) to see
+if your bug has already been reported.
+
+If you're confident that you've found a real bug (or even if you're
+not sure!) please go ahead and [file an issue on
+GitHub](https://github.com/chocolate-doom/chocolate-doom/issues/new).
+You'll need a GitHub account, but it's pretty easy to sign up.
+
+Please try to give as much information as possible:
+
+* What version of Chocolate Doom are you using? Check the title bar of
+  the window for the version number.
+
+* Chocolate Doom runs on many different operating systems (not just
+  Windows!). Please say which operating system and what version of it
+  you're using.
+
+* Please say which game you're playing (Doom 1, Doom 2, Heretic,
+  Hexen, Strife, etc.) and list any add-on WADs you're using. Please
+  mention if you have any special configuration you think may be
+  relevant, too.
+
+### Feature requests
+
+Chocolate Doom is always open to new feature requests; however, please
+be aware that the project is designed around a deliberately limited
+[philosophy](../PHILOSOPHY), and many features from other source ports
+will not be accepted. Here are a few common requests which are often
+rejected:
+
+* "High resolution" rendering (greater than 320x200 display).
+
+* An option to disable Vanilla limits, such as the visplane rendering
+  limit.
+
+* Ability to play "No Rest For The Living", the expansion pack which
+  comes with the XBLA / BFG Edition of Doom.
+
+But if you're not sure whether your feature is in line with the
+project philosophy, don't worry - just ask!
+To make a feature request, just [file an issue on
+GitHub](https://github.com/chocolate-doom/chocolate-doom/issues/new).
+
+### Bug fixes / code submission
+
+Thank you for contributing code to Chocolate Doom! Please check the
+following guidelines before opening a pull request:
+
+* All code must be licensed under [the GNU General Public License,
+  version 2](https://www.gnu.org/licenses/old-licenses/gpl-2.0.en.html).
+  Please don't reuse code that isn't GPL, or that is GPLv3 licensed.
+  Be aware that by submitting your code to the project, you're agreeing
+  to license it under the GPL.
+
+* Please follow the coding style guidelines described in the
+  [HACKING](../HACKING) file.
+
+* The guidelines given above in the "feature requests" section also
+  apply here. New features which aren't in line with the project
+  philosophy are likely to be rejected. If you're not sure, open a
+  feature request first and ask before you start implementing your
+  feature.
+