Update CI instructions with git fetch and git config

[jayqi]

Hello, I've recently been experimenting with getting mike to work with GitHub Actions, and I have some suggested improvements to the README.

• I found that manually calling git fetch for just the head commit on the gh-pages branch was good enough. I think this is preferable to setting fetch-depth: 0, which can lead to a lot of unnecessary data transfer for repos with a large disk footprint, a long history, and/or many branches.
• The need to configure the git user was undocumented. I've seen examples in various issues using git config --global, but actually just git config for the current repository was enough.

My example for reference.

Thank you for creating and maintaining this nifty tool!

[jimporter]

That does seem like a cleaner way to do things, thanks! I've merged this with a few small editorial changes (but since I did it on the command line, Github didn't notice that this was actually merged).

Resolved via 87a6393ab15d016cf7dbbfa47fccf987bfb46e05.