[electrum] README.md follow-ups

Summary:
D14250 broke the setup.py script and missed the needed update in MANIFEST.in

Fix it.

Test Plan:
Make sure the setup.py script now works, build the source package and check that it contains README.md:

cd electrum
python setup.py --version
contrib/build-linux/srcdist-docker/build.sh && tar tvf dist/ElectrumABC-5.2.5.tar.gz | grep README.md

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14258