add rust-toolchain file

[mike-kfed]

Thanks for the easy to use template! How do you feel about adding a file called rust-toolchain with content

[toolchain]
channel = "nightly-2021-01-07"
components = ["rust-src"]

afterwards simply running cargo build does the trick, and it would help with the current Rahix/avr-hal#124 issue (and generally down the line as AVR support will stay nightly for long)

[HKalbasi]

Thank you for this tip! Is it know OK?

[mike-kfed]

the file needs to be called rust-toolchain not rust-toolchain.toml

once the rename is done this issue can be considered fixed, thanks!

[HKalbasi]

I thought it is ok to use .toml when I see rust-lang/rustup#2653 merged

But it is too new :(