[TASK][DOC] Add paragraph about the reStructuredText Primer

A document "reStructuredText Primer for TYPO3 Users" has been created
on GitHub and gives a brief introduction to reST concepts and syntax.

It should be mentionned in this manual.

Change-Id: I875372abab0714b34c5224ec8025cf91de145954
Reviewed-on: https://review.typo3.org/28403
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers