Extract contribution guides into CONTRIBUTING.md

[iamandrewluca]

Would be better to move all instructions how to setup project and contribution to a separate file CONTRIBUTING.md. PR on me if green light on this.

[roata]

I would prefer to leave it as is, because it is not a long document, and it does not yet suffer from readability issues that would be resolved by breaking it down into pieces. But most important - it is short, and whoever reads it can feel confident that they've seen all there is to see.

However, if you take it upon yourself to do this, then:

• keep in mind that the the current readme is in rst;
• the current doc should contain a link to the newly minted section (so the reader can "flow" from one doc to the next, without having to explicitly look for it);
• add other sections that are necessary (e.g., the data structures and the format of CSV files, the code of conduct - these are candidates for another section);
• think of what sections will be needed in the future.