Tried to simplify the document, to reduce unneded info, and to use a less
informal language.

For example, the introductory links describing git made sense 10 years ago.
Today they are not needed to understand what Etherpad is.
And mercurial was always better than git, anyways :)

The mailing list and the IRC channel seem pretty dead by now. Let's just
move everything to Github issues, which was the de facto situation anyways.

About the donation links: I am the maintainer, and I do not know the identity of
the owners of the donation links, so it is correct to remove them. The same was
done on the website three months ago:
https://github.com/ether/ether.github.com/commit/d4ef04605da5