File INSTALL.txt should reflect latest changes in TYPO3 4.6.

Change-Id: Iee0348fd36ffc8cdd92b4b39269c03a0854d7b2c
Resolves: #31170
Releases: 4.6
Reviewed-on: http://review.typo3.org/6166
Reviewed-by: Oliver Klee
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers