Clarify Rust requirement
Should anyone already have rust installed on their system, it is entirely possible that it would be out of date. A note that mentions the use/requirement of rust-nightly would help those users know that they might need to uninstall an older version first and would prevent the install of any older version that cannot be used. In the one case such a note would eliminate wasted time and effort regardless of the method used to obtain rust-nightly.
Thank you.