Add developer guide to CONTRIBUTING.md

Summary:
Explain the development process we want, and how
to work with the project's repository and tools.

Test Plan:
Read the guide, make changes based on feedback
from people trying to follow the steps

Reviewers: deadalnix, schancel, freetrader, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: nbelakovski, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D750