Split README into several parts

[viktormalik]

The README has become quite long so it makes sense to split it into several documents (preferably in a docs/ folder).

This consists of several parts:

• [x] #296
• [x] #297
• [ ] #298
• [ ] #299

Feel free to suggest more ideas for enhancing documentation. Once it starts being large, we can come up with a more sophisticated system (e.g. GitHub pages or Read the Docs).