Lang-team RFC guidelines appear to be out of date

[ehuss]

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

May I suggest that the lang_changes.md file be deleted and change the RFC link to point to https://lang-team.rust-lang.org/how_to/propose.html, which appears to be more up-to-date on your processes? However, I'll leave it up to ya'll on how you would best like to improve it.