CONTRIBUTING.md: write contributing guidelines for GitHub

CONTRIBUTING.md

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+# Contributing to Limnoria
+
+When you send pull request, please send it to the testing branch. It's merged to master when it's considered to be enough stable to be supported.
+
+Don't fear that you spam Limnoria by sending many pull requests. According to @ProgVal, it's easier for him to accept pull requests than to cherry-pick everything manually.
+
+See also [Contributing to Limnoria] at [Limnoria documentation].
+
+[Contributing to Limnoria]:http://supybot.fr.cr/doc/contribute/index.html#contributing-to-limnoria
+
+[Limnoria documentation]:http://supybot.fr.cr/doc/index.html