docs(readme): rewrite for this temporary fork

Document that this is a temporary tweakcc fork while upstream catches
up. Lists the base version (4.0.11 at upstream 2e1d03e / 2.1.113), the
four cherry-picked open upstream PRs (Piebald-AI#601, Piebald-AI#646, Piebald-AI#655, Piebald-AI#664) with
credits, and the four fork-specific fixes added on top (scoped
backslash-doubling, backtick test realignment, 2.1.113 regex updates,
verbose-destructure guard). Points readers at the upstream README for
full docs.

Author: Ben