Add README with initial documentation

[lukaszb]

Before writing any more code we should try to explain to users how to use this library.

We'd need to re-read it and make sure it's simple enough.

[piotrekno1]

I like the high-level overview for CQRS+ES presented in the Axon docs:
url. I can imagine that sth similar could be useful here apart from low-level details.