change add to library to add to my music

Change in readme.md "Next, select a random song on Apple Music you don't have in your library yet, right click and choose 'Add to library'. " to "Next, select a random song on Apple Music you don't have in your library yet, right click and choose 'Add to My music'. " to make it more clear what to do.

I've submitted a pull request with this change in it. Hopefully Simon accepts it.