Initialize contribution guidelines

[ikoevska]

This is a very generic contribution guide. I recommend creating an additional document with information for developers, such as dev environment setup, coding guidelines, etc. (I'll open an issue for that.)

I've placed the document in the hidden .github folder where we can also place an issue template and a pull request template. Let me know if you'd like me to move the document in the root.

[pavelpenchev]

@jortegac @zeridon can you please review