Add contributing.md file

Author: pavelnikolov

CONTRIBUTING.md

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+## Contributing 
+
+- With issues:
+  - Use the search tool before opening a new issue.
+  - Please provide source code and commit sha if you found a bug.
+  - Review existing issues and provide feedback or react to them.
+
+- With pull requests:
+  - Open your pull request against `master`
+  - Your pull request should have no more than two commits, if not you should squash them.
+  - It should pass all tests in the available continuous integrations systems such as TravisCI.
+  - You should add/modify tests to cover your proposed code changes.
+  - If your pull request contains a new feature, please document it on the README.