[TAKS] Document how to create custom postProcessors in FORM

This patch adds documentation about creating custom postProcessors.

Resolves: #75963
Releases: master, 7.6
Change-Id: Ia0bb2b5d33cf30cba931953088c402307cc7ee40
Reviewed-on: https://review.typo3.org/47951


Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>