Add a line about reviewing a Diff

Summary: Add a line about reviewing a Diff, to help people not familiar with arc.

Test Plan: Read the line

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

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1002