Add documentation for "update" and "scale" commands

[Lutz Hühnken]

Some senza commands are not mentioned in the documentation / README. Namely, I noticed "update" and "scale" are not being mentioned at all, although they seem quite relevant.

[Henning Jacobs]

Feel free to do a PR, there is also the docs page here: http://docs.stups.io/en/latest/components/senza.html