[ANNOUNCEMENT] update to README.md wrt contacts and contributions

[Ondrej Lichtner]

Hi all,

just a small announcement:

* due to the freenode situation I have removed the info about the #lnst
  channel on freenode and I will not be joining it anymore. As the
  channel wasn't really active anyway I have not created a replacement
  channel anywhere either. As such if you want to contact us, please do
  so over github issues or via this mailing list.

* at the same time I decided to try out accepting pull requests and
  issues for contributions. For now I still want to keep the commit
  history linear so any reviews I'll do will mention this and I'll
  enforce it. But this may change in the future.

* I also added a link to the read the docs documentation page. It's not
  "complete" but should be somewhat useful already.

Thanks,
-Ondrej