[TASK] README.rst should allow referencing files in Resources/

In order to be even more user-friendly, images should be allowed to
be used in a simple README.rst manual.

Change-Id: I4b04589abfd574e97ac4a5f1045422ecd13ba450
Resolves: #54815
Reviewed-on: https://review.typo3.org/26683
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers