[README] Updated comments on stacked diff procedure

Summary:
Update CONTRIBUTING.md to reflect current procedure for stacked diffs

I went through the blog posts and hit these roadblocks.

Test Plan: Confirm the comments are legible & make sense.

Reviewers: #bitcoin_abc, emack, sdulfari

Reviewed By: #bitcoin_abc, emack, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11591