Describe how to swap in local modified crate

[pnkfelix]

Recently while working on rustc I mused that a certain task would have been easier if I could have plugged in a different version of the ena crate.

It is probably easy to do this in general. @nikomatsakis says they do it by modifying Cargo.toml in an appropriate fashion. Perhaps even a patch there would work.

But in any case, it would be good to document the process.

[mark-i-m]

Does this answer the question: https://internals.rust-lang.org/t/how-to-specify-a-private-repository-for-one-library-like-libc/9779