readme should be updated

[MichaelB7]

Also would be good to add a little description of each book. If I get some time, I will start a PR. May be ideal for a little collaborative effort.

[snicolet]

Hi Michael :-)

Would you still be interested in helping with the Readme for the book directory?

We are currently consolidating many tools used for developing Stockfish under the
official-stockfish umbrella on Github, and having a well documented book directory
would be a nice bonus for the project.

[C8RRUPT]

Ye