Add contribution files

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CONTRIBUTING.md

+People who have agreed to the
+[Cesanta CLA](https://docs.cesanta.com/contributors_la.shtml)
+can make contributions. Note that the CLA isn't a copyright
+_assigment_ but rather a copyright _license_.
+You retain the copyright on your contributions.
+
+We follow the Google C/C++ style guide: https://google.github.io/styleguide/cppguide.html
+We'd appreciate if your contribution follows the same style guide.