Clarify how accessibility whitelisting works on macOS

[dessant]

The wiki should be updated to document how to whitelist the client app to control the mouse and keyboard on macOS.

It's not yet clear if it's enough to whitelist the browser and permissions are inherited, or buster-client must be explicitly whitelisted, and what happens when the browser or buster-client are updated. This comment has some helpful details regarding the issue: #139 (comment)

https://github.com/dessant/buster-client/wiki/Installing-the-client-app#macos

[dessant]

I do not have a Mac to test, so if anyone knows how whitelisting works, please share the details.