[RUST] Missing info and/or update needed to readme I think

[deanm0000]

I'm no compiler expert but I had a hard time getting mold going for compiling a rust project

I finally got it to work and these are the things that I think might help future people

1. I, seemingly, had to install clang. Should that be in the install-build-deps.sh?
2. In the "How to use section" under "A classic way to use mold" it says to pass -fuse-ld=mold but in the "If you are using Rust" subheading, it says link-arg=-fuse-ld=/path/to/mold but it didn't work with the full path. I ended up adding ppa:ubuntu-toolchain-r/test and updating gcc and then passed -fuse-ld=mold in my config.toml.
3. Having the snippet to put in config.toml twice was a bit confusing. Maybe take one out but keep the note about putting it in the home directory for global setting and project one if they don't want it to be global.

[Paulo-21]

For me the doc is clear, what can be done in my opinion is to specified from which version of gcc it recognize the option -fuse-ld.