Docs: request a simple doc about to create/extend `register definitions` (rules and meanings)

[beyonlo]

In my opinion, is very important, to prevent errors, to have a simple doc (with examples) to explain two things:

1. What is rules on the register definitions, I mean, what and how each argument need to be constructed.
2. What means (what it will affect) each argument. Example: until now is not very clear to me what means the len, like as 'len': 1. The reason that is not very clear yet is that some times I changed the len to other value, and nothing change. Or because my register definitions is not correct - so, is a one more good reason to have a doc explaining in details about register definitions.

[brainelectronics]

found by #10

[brainelectronics]

Relates to #31