Update developer notes and link to it from contributing docs

Summary: As per title.

Test Plan: Read the documentatino

Reviewers: deadalnix, jasonbcox, mbhinder, jimpo, matiu, Mengerian, #bitcoin_abc

Reviewed By: jasonbcox, jimpo, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1298