Update Documentation on how to provide a Custom Scalar

Submitted by Christoph Hertzberg @chhtz

Assigned to Nobody

Link to original bugzilla bug (#1677)
Version: 3.3 (current stable)

Description

We could implement a very trivial custom scalar which essentially just wraps a float and make sure that this implementation is sufficient to be used by all modules.

I will close this PR, since this essentially was caused due to this lacking documentation:
PR-363

Blocking

#1608