Added a note about preserving binaries for signing after doing a gitian build

Summary: This step was not clear and I wiped out my binaries after each step.

Test Plan: Tried it myself

Reviewers: deadalnix, schancel, Mengerian, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1657