Add more usage hints in readme

[Jln Wntr]

Please add more information on how to actually run the python-server!
Example:
$ saltyrtc-server serve gives me:

It is REQUIRED to provide a SSL certificate and a server permanent key unless the environment variable 'SALTYRTC_SAFETY_OFF' is set to 'yes-and-i-know-what-im-doing'

[Lenny]

Have you checked the Getting Started page of saltyrtc.org?

[Jln Wntr]

No! :D But why don't we add these info to the readme?
Thanks for the link though.

[Lenny]

I don't like duplicating stuff but we could sure add a link to that page.