Create CONTRIBUTING.md document

[gund]

So that everybody may know that contributions are welcomed =) and they can see what are the basic rules to follow to make our life easier and contributed PRs merged.

Some of the things to mention in the doc:

• [ ] Semantic releasing is in use
• [ ] Unit tests are run before any commits
• [ ] Code coverage check also
• [ ] Linting as well
• [ ] Commitizen-style commits are essential

Probably it's a good idea to resolve this along with testing investigation so that no one will give up trying to commit something :)

[gund]

I think that I also will include .clang-format config file in order to promote some code formatting assistance and maybe even run that tool automatically so that all code-style will be the same.