Added "flip-link" dependency

[DrZlo13]

Currently "microgroove_app" builds will error:

>cargo embed
   Compiling microgroove_app v0.3.0 (/Users/drzlo/work/sound/microgroove/firmware/microgroove_app)
error: could not compile `microgroove_app` (bin "microgroove_app") due to 2 previous errors
error: linker `flip-link` not found
  |
  = note: No such file or directory (os error 2)

error: aborting due to 1 previous error

       Error Failed to run cargo build: exit code = Some(101).

This PR adds "flip-link" to the prerequisites in readme.