55d17f2c18 Instruct devs to use nightly for embedded (Tobin C. Harding)
Pull request description:
The embedded test crate requires usage of the nightly toolchain, fix the docs to show this.
ACKs for top commit:
apoelstra:
ACK 55d17f2c18
Kixunil:
ACK 55d17f2c18
Tree-SHA512: 5d3d611ff4331d8475f77d260ebaa3da3ae71960b709eb4603056d7034885306b8bcc39be488f3391697d47a1074a695979fb50b6cb31af414f873da8e82bdbd