[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

typo3/contrib/jquery/README.txt

@@ -0,0 +1,6 @@
+Please *DO NOT* reference/use the jQuery-libraries in this folder from your
+own extensions. They are used throughout the TYPO3 CMS core but may be renamed
+or removed without further notice in later TYPO3-versions.
+
+We plan to provide and promote a central API for using jQuery (also in various
+versions) in one of the next releases.