Add documentation

[wrznr]

A note on installation and invocation of your tools would be very helpful!

[maciejjan]

As this repo is a mirror of our internal Gitlab repo, the issue numbers do not match (so the commit above does not reference this issue). Sorry for the inconvenience.

Documentation is now available for the plaintext interface (in the README) - please let me know if you have any feedback. The PAGE interface is still work in progress.