Updates contributing doc with required version of Rust.
Updates contributing doc with required version of Rust.

mattico/rusoto,mattico/rusoto,noggleboggle/rusoto,DualSpark/rusoto,rusoto/rusoto,DualSpark/rust-aws,DualSpark/rusoto,rusoto/rusoto,SecurityInsanity/rusoto,mattico/rusoto,ford-prefect/rusoto,DualSpark/rusoto,ford-prefect/rusoto,cmsd2/rusoto,DualSpark/rust-aws,mthjones/rusoto,indiv0/rusoto,DualSpark/rust-aws,rusoto/rusoto,SecurityInsanity/rusoto,mthjones/rusoto