Add CONTRIBUTING.md

[4rgon4ut]

Adding more direct style/flow instructions for contributors will be beneficial for the project. Current README is quite direct and clear but it lacks some flow bits, proposed document intended to add this missing information and simplify contributions for newcomers:

Guide:
https://gist.github.com/4rgon4ut/e885ae04dddc0dfb632e8597d70494ed
Useful optionals listed in OPTIONALS.md and may be added if necessary

I was trying to not duplicate readme so guide is pretty simple and it would be nice to hear some thoughts from maintainers.
I have some questions:

1. Should we enforce issue creation? ie. creation of issue is always required before the PR
2. I see no versioned release yet, should we add versioning mention? (semver?)
3. Your overall thoughts on styling the the doc (branching/commits convention)

This may probably be simplified even further and become a part of a README btw