[TASK] Improve README.md, add CONTRIBUTING.md

Streamline README.md a bit and add a CONTRIBUTING.md
to hint for basic contribution guidelines.
The CONTRIBUTING.md is especially helpful if the GitHub
repository is forked since GitHub will add a notice with
a link to this file.

Change-Id: I2ff78f72346da4146f6d68777f263d34e0a30a7f
Resolves: #68447
Releases: master
Reviewed-on: http://review.typo3.org/41791


Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>