Add Readme and Contributing guidelines.

Add Readme and Contributing guidelines.

For people which just have found our source code especially over GitHub
sometimes seem to have problems finding out how to contribute. This is
sad as it prevents us building a bigger community. While we have a wiki
which conatins a lot informations, this seems to get overlooked. I guess
a contributing fact is that most software today has these informations
contained in the repository itself. While I am not going to duplicate
the wiki to have these informations in our repository I still think it
is a good idea to have a short guide to lead new contributers into the
right direction.

Someone now could argue that new people just a too * to look at the
right place and that's why we do not need these documents. I do not
think so because of several points:

First people get used to look for information at a readme. I guees that
is because of the fact both GitHub and Gitlab place the readme so
prominently.

Second starting in a new project is hard. Finding the right place where
to start is also not easy as every project is different. Giving people a
short introduction what the first steps could be is definitely needed.

Signed-off-by: Jonatan Schlag <jonatan.schlag@ipfire.org>