Add in example for how to use with Mill.

[ckipp01]

This just adds a small example of how to include this for mill-scalafix users right underneath the example for sbt. Figured it might be useful for someone if they aren't familiar with how to add it.

[codecov]

Codecov Report

Merging #102 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #102   +/-   ##
=======================================
  Coverage   90.67%   90.67%           
=======================================
  Files           4        4           
  Lines         236      236           
  Branches       13       13           
=======================================
  Hits          214      214           
  Misses         22       22           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f055d52...168bcbb. Read the comment docs.

[liancheng]

Thank you!