Add contributing page for Github

CONTRIBUTING.md

+We greatly appreciate your support and contributions and they are always welcomed!
+
+Github pull requests are the convenient way to contribute to OpenCV project. Good pull requests have all of these attributes:
+
+* Are scoped to one specific issue
+* Include a test to demonstrate the correctness
+* Update the docs if relevant
+* Match the [coding style guidelines](http://code.opencv.org/projects/opencv/wiki/CodingStyleGuide)
+* Don't messed by "oops" commits
+
+You can find more detailes about contributing process on http://opencv.org/contribute.html
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