Adds extra info about python-marconiclient and usage instructions

Extends the readme with information about what is included in
the python-marconiclient package and adds some usage instructions
for the API (link to the existing wiki page) and the CLI

Change-Id: I6c5a588d85af53237b3cb11bb9cc8b6e31472fd1
Closes-Bug: #1350867
Merge "Adds extra info about python-marconiclient and usage instructions"