[FEATURE] Followup: Introduce read-only column for file mounts

Added missing feature document for http://review.typo3.org/31096

Resolves: #59830
Releases: master
Change-Id: Id71c579e7e12ba6ad6a58cdae949f9165bb1bc60
Reviewed-on: http://review.typo3.org/32995
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>