Duplicate the rust-toolchain file so the crate can be published (#51)

- This is unfortunate, but the crate must include a copy of the rust-toolchain file since it needs to build ROMs with this version.
- When updating the toolchain file, both copies need to be kept in sync.
- Maybe it is worth writing a build script that will ensure they are identical...
  - Sounds challenging! The build script must work when building from the repo and when building in isolation (downloaded from crates.io) It would have to be at least smart enough to figure out which scenario it is building in.
  - I'll defer that until updating the duplicate toolchain file becomes a problem.

Prepare v0.1.2 release (#16)

Fix README path

Add package name to help text