Update readme to make it clear this project isn't maintained

[losinggeneration]

Looking at older issues to see if/when anyone mentioned Firefox Quantum support, @stevepeak closed #50 with the comment that this repo isn't likely to be updated. That should be made clear in the readme instead of being obscured through a closed comment.

[rsyring]

It's too bad b/c this extension is really helpful. IMO, Codecov would be better served by marketing the extensions. It's a huge time saver when looking through a GH PR to not have to flip to another browser tab to know what has been covered.

[ntwb]

It should also be removed the docs here https://github.com/integration/codecov

I just read the above, installed the extension, read this, uninstalled the extension 🤷‍♂️

[willdurand]

Hi @stevepeak, is this extension still maintained? I found the link in the "new GH integration" and, surprisingly, the extension is not compatible with FF 57+. It says "I'm waiting on Mozilla staff to allow me to update the app...", which is something I can help with.

[adamjstewart]

The https://codecov.io/ website should also be updated to make it clear that there is no browser extension supported.

This browser extension was the main reason we switched from Coveralls to Codecov for Spack. It's a shame to see such great software disappear...