Document minimum supported rust version

[CosmicHorrorDev]

Sadly from what I can tell there's no way to provide information about the minimum supported rust version in Cargo.toml, but there have been a number of issues raised here because of people using outdated compilers and getting build failures (#107 #66 etc.). If a minimum supported rust version was documented in the README and tested against with CI then hopefully people will understand the issue earlier.