Adding missing setup steps to developer-notes to ensure new users can push changes upstream

Summary: I ran across some issues following the instructions in developer-notes, so I added some instructions to resolve them.

Test Plan: N/A

Reviewers: deadalnix, O1 Bitcoin ABC, #bitcoin_abc, schancel

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc, schancel

Subscribers: schancel

Differential Revision: https://reviews.bitcoinabc.org/D720