CLOSE this repo or add message that it's not the official repo

[MichaelCurrie]

This repo is NOT the official repo for openpyxl despite that it is the top google hit.

As noted on the PyPI site: https://pypi.org/project/openpyxl/,

The actual source is here: https://foss.heptapod.net/openpyxl/openpyxl

This repo hasn't been updated for 8 years, since 2015. It should be shut down as people are still mistaking it for the official repo and leaving issues.