README.md (and thus rust docs) are stale

[rbtcollins]

Looks like #[rustversion::any....] is the right syntax now, but the published docs still claim #[rustc::nightly]... etc

[rbtcollins]

Argh no, its different.

rustc-select still exists and has googlejuice.

Perhaps do a 0.1.3 of rustc-select that will fail to compile and point people at rustversion?

[dtolnay]

I yanked all the select-rustc releases because rustc has refused to compile them for a long time (#6).