[Feature Request] Document using the ab_glyph feature

[Monadic-Cat]

What is the feature ?

Recently, #311 was merged, and added a new text rendering backend feature.
It would be nice if there were documented instructions on how to use it, especially when the docs.rs page for plotters does not show the extra function exported for the purpose (because cfg work like that, lol).

Why this feature is useful and how people would use the feature ?

While it is now possible to use ab_glyph for text rendering in plotters, I can't find anywhere that is explained outside of browsing the project source code. It would be really nice if the feature were called out, especially for cross-compiling use cases where it's much easier to use ab_glyph, as opposed to manually putting together a cross sysroot to make font-config cross compile correctly.

Additional Information

I'd be happy to write this documentation if someone could tell me what places would be most appropriate for it.

[AaronErhardt]

I'd be happy to write this documentation if someone could tell me what places would be most appropriate for it.

I think this should be the right place to add it: https://github.com/plotters-rs/plotters/blob/master/doc-template/readme.template.md#list-of-features

I'm not entirely sure if there are other places where features are advertised, but if you find something you can add it there too.