[TASK] Integrate default README.txt

This file is a modified and updated version like it has been
releases with every package in the past. Since these files have
been taken from git.typo3.org/TYPO3CMS/Distributions/Base.git,
which is target to be cleaned up, the file is explicitely put
to old branches as well.

Resolves: #57656
Releases: 6.1, 6.0, 4.7, 4.6, 4.5
Change-Id: I3b696895deaf03b2f630e12f1bd7b17b649b985c
Reviewed-on: https://review.typo3.org/29174
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader