Create CONTRIBUTING.md

Author: probonopd

CONTRIBUTING.md

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+# Contributing
+
+We love pull requests from everyone. 
+When contributing, please consider to join us on IRC. We are on __#AppImage on irc.freenode.net__.
+
+Fork, then clone the __appimagetool/master__ branch of the repo:
+
+    git clone -b appimagetool/master --single-branch --recursive [email protected]:your-username/AppImageKit.git
+
+Compile:
+
+```
+cd AppImageKit/
+sudo bash -ex install-build-deps.sh
+bash -ex build.sh
+```
+
+Make your change. Build again to be sure you didn't break anything. Run the resulting binaries to test them.
+
+Push to your fork and [submit a pull request][pr].
+
+[pr]: https://github.com/probonopd/AppImageKit/compare/appimagetool/master
+
+At this point you're waiting on us. We like to at least comment on pull requests
+within three business days (and, typically, one business day). We may suggest
+some changes or improvements or alternatives.