Moved test/functional/README to doc

Summary: Depends on D1496. Completes T357: This doc file is not easy to find with grep, as it does not reside in the doc directory with most other docs. This change fixes that and updates developer-notes and functional-tests to match.

Test Plan: read it

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1497