Merged /test/functional/README into functional-tests

Summary:
There was a goof in D1497 where I said I moved /test/functional/README but actually moved /test/README.
This diff merges /test/functional/README with what was /test/README turned functional-tests.md.
Completes T357, whereas D1497 actually completed T356.

Test Plan: read it

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1503