handbook/cutting_edge: document the n-number

Document how to convert a git hash into a 'n' number. I've been asked
this a few times and it would be good to have an easy, linkable answer.
It seems to fit best here.

Feedback from: markj@, Pau Amma
Reviewed by: erj@, ceri@
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30938