Added note for Mill users

[davesmith00000]

Added a brief note to help direct Mill users who would like similar functionality.

Feel free to use or not or let me know if you'd like changes, just trying to add a sign post of the next traveller. 😄

[DavidGregory084]

Before this is merged, I just wanted to drop a note that I'm thinking about archiving mill-tpolecat and signposting scalac-options instead as mentioned on the mill-tpolecat PR: https://github.com/DavidGregory084/mill-tpolecat/pull/54#issuecomment-2410493258