better docs

[ThomasWaldmann]

quickstart section misses quite some infos.

[ThomasWaldmann]

pull request #36 adds some docs for config file usage, but it has only the most fundamental params.

[ThomasWaldmann]

some ideas about the README.rst:

I think there is too much stuff in it right now. People who are deeply interested shall read the documentation, not just the readme. But the readme's task is to GET people interested within a minute.

Like "what is dyndnsc and why do I want to use dyndnsc rather than some similar software?".

This stuff can be included as "about" section into main documentation btw, so one can avoid any duplication.

See there at the bottom for an example what I mean: https://github.com/nsupdate-info/nsupdate.info

[infothrill]

note to self: remove list of compatible services and point users to --list-presets. Also document how to add new presets.

[infothrill]

Not perfect but loser than before. Feel free to add more compelling text.