README: Add multirust as alternative rustc installation method
README: Add multirust as alternative rustc installation method

multirust facilitates installing multiple rustc/cargo versions without
having to handle PATH/LD_LIBRARY_PATH by hand.

google/tock-on-titan,lizardo/tock-devel,lizardo/tock-devel,google/tock-on-titan,lizardo/tock-devel,google/tock-on-titan