docs: add a note for macos users to get rid of anki connection errors

[thelazyoxymoron]

For MacOS users, they might see the following error when trying to do anki export:

Error: Could not connect to Anki-Connect

Caused by:
    0: error sending request for url (http://localhost:8765/): connection error: Connection reset by peer (os error 54)
    1: connection error: Connection reset by peer (os error 54)
    2: Connection reset by peer (os error 54)

This can be fixed by disabling the "App Nap" for Anki. The PR adds a note in the README on how to do that.