Minor: Explain directories

[DazWilkin]

The docker-compose init service creates a directory called default that may then be queried:

curl --insecure https://localhost/v1/directories/default

Subsequent keytransparency-client post commands implicitly use this default directory.

It was only through some experimentation that I realized, it's easy to create other directories and these may be referenced using the directory flag on the post command.

Recommend
Adding some clarity to the README.md about directories and how to use them.
Perhaps, with an example of adding a second directory and then POSTing to it?