Add usage section in ockam_commad README

[lameferret]

Right now, command usage instructions are all over the place.
Every possible way to use CLI can be consolidated into ockam_command/Readme. It will be quite a lot, but would be required only once, as later on it can be updated with every PR.

It will be the single source of info, saving time by

• being an instruction reference to be provided with issues making changes in CLI. example(current behaviour)
• not bothering devs to provide instructions to test changes. example
• removing over head time to wait for devs to provide instructions.