Instruct devs to use nightly for embedded

[tcharding]

The embedded test crate requires usage of the nightly toolchain, fix the docs to show this.

[apoelstra]

ACK 55d17f2

[Kixunil]

ACK 55d17f2

[Kixunil]

I wonder why nightly is needed though, I was under the impression this should work on stable at least.

[tcharding]

I didn't dig into it but this is the error

error[E0554]: `#![feature]` may not be used on the stable release channel
 --> /home/tobin/.cargo/registry/src/github.com-1ecc6299db9ec823/linked_list_allocator-0.10.4/src/lib.rs:1:41
  |
1 | #![cfg_attr(feature = "const_mut_refs", feature(const_mut_refs))]

[Kixunil]

Ah, OK, I think we just should switch the dependency to something more sensible. IIUC that feature is quite unstable.