Reason for requiring `nightly`

[andy-preston]

Because I'm a Rust newbie (but have been using other compilers for decades), I'm nervous of requirements to use nightly. Would it be possible to update the README to include a brief reason why this crate requires nightly?

[kiffie]

Yes, I could add something to the README.

The main reason for nightly is that the core and alloc libs must be compiled by cargo for the mipsel-unknown-none target. Stable cargo cannot do such compilations.The pic32 crates should not use experimental language features, except maybe some inline assembly, which is still experimental for MIPS.