Libs-team RFC guidelines appear to be out of date

[ehuss]

The guidance at https://github.com/rust-lang/rfcs/blob/master/libs_changes.md for filing RFCs appears to be quite old and doesn't contain all the detail about how the libs team seems to operate these days.

Primarily, it doesn't seem to recognize the ACP process, which is what I assume how you would like to process most changes these days. I actually don't understand when someone should use an RFC versus an ACP, and I don't know how to find that out. I'm also uncertain what else in that document might no longer be current.

Would someone be willing to update the guidance (at least adding a link to something describing ACP), or point the link somewhere that better reflects on how libs changes should be proposed?