Make the docs more clear that setting your git remote origin is necessary for Arcanist to work properly

Summary:
New users tend to encounter this when they clone from github. While we did improve the CONTRIBUTING.md docs, it could be more clear that
setting the remote is necessary for Arcanist to work properly.

Test Plan: N/A

Reviewers: schancel, movrcx, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1195