Update developer notes to include Arcanist setup instructions

Summary: I didn't realize I needed to use Arcanist when submitting changes for my first diff, and I found that I couldn't commit them. Nothing in the ABC docs indicated this, so I'm adding them.

Test Plan: N/A

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Mengerian, deadalnix, freetrader

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Mengerian, deadalnix, freetrader

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D648