[libc++] Document procedure for adding officially supported platforms

[philnik777]

We have a few requirements for official support of platforms, but we don't actually document them anywhere so we can direct people to a page informing them of the requirements and how to achieve them. This includes the requirement that there is a CI configuration (which we do document at https://libcxx.llvm.org/#platform-and-compiler-support, but that's not very visible IMO), having a contact person for the bot and platform and I'm sure there are more things I can't remember right now.

[ldionne]

I agree, this could be documented more clearly.