[TASK] Add README for jQuery-libraries in contrib-directory

Those libraries are used through the core but should not be
referenced/used from outside (for example foreign extensions)
as files may be renamed or removed without further notice.

Change-Id: Ifea45d74a19e0cd6cbc1bb555ae40fd8f9ad8cde
Resolves: #52844
Releases: 6.2
Reviewed-on: https://review.typo3.org/24791
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind