Added CONTRIBUTING.markdown

Author: pedrolamas

CONTRIBUTING.markdown

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+Follow these guidelines, in no particular order, to improve your chances of having a pull request merged in.
+
+ * Make each pull request atomic and exclusive; don't send pull requests for a laundry list of changes.
+ * Even better, commit in small manageable chunks.
+ * Tabs, not spaces. Bracket style doesn't matter.
+ * Changes to XmlDeserializer or JsonDeserializer must be accompanied by a unit test covering the change.
+ * No regions except for license header
+ * Code must build for .NET 3.5 Client Profile, Silverlight 4 and Windows Phone 7
+ * If you didn't write the code you must provide a reference to where you obtained it and preferably the license. 
+ * Use autocrlf=true `git config --global core.autocrlf true` http://help.github.com/dealing-with-lineendings/