minimal supported version of Rust missing from Contributing section

[zdenek-crha]

The 'contributing' section of the readme contains note that the getopts crate is used by compiler and contributors must be careful to not break anything. Is there also consensus on what is the minimal supported version of rust compiler used to build the crate?

Example: I want to contribute by writing example code, but I'm not sure if using std::maches!() macro is a good idea or not since it is supported only from Rust 1.42.0.

I have err-ed on the side of caution and used different approach. But it would be nice to know what version to target so that example code do not use outdated idioms.