Added simple guide for contributions.

CONTRIBUTING.md

@@ -0,0 +1,49 @@
+Bug reports, feature requests or code contributions are always very welcome.
+To make things easier, here are a few tips:
+
+Reporting bugs, requesting features
+-----------------------------------
+
+- Best way to report bugs and request new features is to use GitHub
+  [issues](https://github.com/mosra/magnum/issues), but you can contact me
+  also any other way.
+
+Code contribution
+-----------------
+
+- Building and installing Magnum is described in the
+  [documentation](http://mosra.cz/blog/magnum-doc/building.html).
+- Follow the project coding guidelines. In short - try to match style of
+  surrounding code and avoid any trailing whitespace. When in doubt, consult
+  coding guidelines, which are available also
+  [online](http://mosra.cz/blog/magnum-doc/coding-style.html).
+- Document your code. When updating or adding new API, make sure that Doxygen
+  documentation is up to date. Run
+
+      doxygen
+
+  in project root to generate the documentation and check that your
+  modifications didn't add any warnings.
+- Build unit tests (`-DBUILD_TESTS=ON` parameter to CMake) and run them
+  using
+
+      ctest --output-on-failure
+
+  in build directory. All tests should always pass. Add new tests or modify
+  the existing to make sure new code is properly covered (if possible). Here
+  is a [short tutorial](http://mosra.cz/blog/corrade-doc/unit-testing.html) to
+  help you with creating unit tests.
+- Best way to contribute is by using GitHub
+  [pull requests](https://github.com/mosra/magnum/pulls) - fork the repository
+  and make pull request from feature branch. You can also send patches via
+  e-mail or contact me any other way.
+- All your code will be released under license of the project, so make sure
+  you (or your employers) have no problems with it.
+
+Contact
+-------
+
+- Website - http://mosra.cz/blog/
+- GitHub - https://github.com/mosra/magnum
+- E-mail - mosra@centrum.cz
+- Jabber - mosra@jabbim.cz