Update documentation with user-agent override

[edevenport]

Changing the user-agent string was added in PR #63. This should be added to the README.

[bdrung]

Note: It is not a user-agent override, but the option to add a user agent. So instead of sending "profitbricks-sdk-python/4.1.0", you can send "client-app/1.2 profitbricks-sdk-python/4.1.0" instead.