Update client installation instruction & credentials management

[JulienPeloton]

This PR brings two new updates:

• New installation instruction for the client. The way to install fink-client is as easy as pip install fink-client.
• New configuration & credentials management using fink_client_register.

Both changes would be effective after the merge of https://github.com/astrolabsoftware/fink-client/pull/55.