Add a description to the client repositories

[Danil42Russia]

If you go into the client repository, it's not clear:

1. How to install the client;
2. There is no example code on how to send basic metrics;
3. No instructions on how to locally bring up an environment to develop the client.

I would like to see all of this in a README file.

P.S. Python and Nginx has this and would like to see it for everyone :)

[flyingmutant]

This would definitely be a good improvement! Until we found time to do this ourselves, patches are welcome!