Add CONTRIBUTING.md and CODE_OF_CONDUCT.md

[fdiblen]

The project doesn't have contributing guidelines or a code of conduct.
For code of conduct we can simply use Contributor Covenant for now.

[kaygraf]

I agree with starting with Contributor Covenant as code of conduct.

Contributing guidelines should be harmonised with CONTRIBUTING from TechRadar

[kaygraf]

Please note CONTRIBUTING.md is part of #55