Set minimum Rust Version

[Quitlox]

Hi there! First of all, great tool! I'm having a lot of fun with it.

While installing flavours on a different machine using cargo build (same using the AUR package), I encountered an error.

As it turns out, just upgrading my Rust version from 1.52 to 1.57 solved the issue.

It took me a second to figure out, since I am not experienced with Rust, and I imagine I am not the only one. Would it be an idea to set a minimum Rust version in cargo.toml? It seems that this is possible as of recent (rust-lang/cargo#9732).

Alternatively, I would propose to add one or two lines in the README.md stating upgrading as a potential fix for build errors. The latter I could do myself, the former is probably easier for someone with experience in Rust 😛.

[Misterio77]

Hey! Thank you for the report!

I'm not sure if all distros have rust 1.57 packaged, so i think i'd like to play safe and enforce 1.56, if possible. Could you check if it compiles with 1.56?

I think both solutions combined would work the best!