Add section on Deform to manual

Here's some documentation to accompany https://github.com/diagrams/diagrams-lib/pull/148  It at least explains the aspects of what's `Deformable` that gave me headaches writing the code.

Is there a way to mark up `Transformation`s that will be linked correctly?  Sometimes the plural is correct in the sentence, but when I place backticks in the obvious place, the .rst parsing shows them in the output HTML, and there's no link.