Added notes in gitian-signing for optionally generating SHA256SUMS

Summary:
We did this for the 0.18.0 release, so I'm recording the process here.
I marked it as optional as this is already being supersceded by signing the binaries
individually so that automatic sig verification can be applied.

Test Plan: I did this for the 0.18.0 release

Reviewers: deadalnix, schancel, Mengerian, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1664