Add developer notes about blocking GUI code and src/interfaces/README.md
Summary:
Partial backport of Core PR 10244 (remaining documentation changes)
https://github.com/bitcoin/bitcoin/pull/10244/commits/996013769711bd507cdcd6dde88cbd59fcd4fbad
https://github.com/bitcoin/bitcoin/pull/10244/commits/ea73b84d2ddde22487dee0f71d7a619051e067f2
Completes T417
Test Plan: Read it
Reviewers: deadalnix, Fabien, #bitcoin_abc
Reviewed By: Fabien, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D2900