Document the list of supported architectures

[Shnatsel]

I could not find any information in the docs on whether Ring provides architecture-independent fallbacks for when an optimized implementation for the target CPU cannot be found, or ships architecture-specific implementations only. And if it's the latter, then support for which architectures is implemented.

[briansmith]

Right now the allowed architecture allowlist is documented in https://github.com/briansmith/ring/blob/main/include/ring-core/target.h and the allowed operating system allowlist is documented in https://github.com/briansmith/ring/blob/main/src/rand.rs. Not ideal but also guaranteed to be up to date.