Update README Antigen instructions

[Blaradox]

The antigen installation instructions on the README seem to be out of date. Antigen seems to still have issues with repositories that don't use "master" as their default branch. Here is the relevant issue.

[belak]

Thanks for submitting the PR - this should be resolved now.

[Blaradox]

Of course, glad to help!

I'm curious you said you haven't used antigen for a while, what do you use instead? Antigen hasn't gotten an update in a long while so I'm open to alternatives!

[belak]

I wrote a small function, belak-load which takes the path to a plugin and clones it if it doesn't exist.

https://github.com/belak/dotfiles/blob/main/zshrc

It's not perfect but it's small and stable enough that it hasn't changed in ages. Adding a way to pin repos to commits would probably be the next step.

[Blaradox]

Oh fantastic, I'll have to check that out! Pinning repos to commits sounds like it would be a great feature.