Add contributing guide [skip ci]

CONTRIBUTING.md

+## Contributing
+
+Thank you for considering contributing to opencv-python.
+
+### 1. Where do I go from here?
+
+If you've noticed a bug or have a question that doesn't belong on
+[Stack Overflow](http://stackoverflow.com/questions/tagged/opencv-python),
+[search the issue tracker](https://github.com/skvark/opencv-python/issues?q=something)
+to see if someone else in the community has already created a ticket.
+If not, go ahead and [make one](https://github.com/skvark/opencv-python/issues/new)!
+
+### 2. Fork & create a branch
+
+If this is something you think you can fix, then
+[fork opencv-python](https://help.github.com/articles/fork-a-repo)
+and create a branch with a descriptive name.
+
+### 3. Make your changes
+
+Make required changes.
+
+### 4. Create a pull request
+
+[Make a Pull Request.](https://help.github.com/articles/creating-a-pull-request)