Add install instructions to README

[toadjaune]

Hi there,

The README mentions that this project relies on pyobjc to work, but doesn't explicitly state that this is a dependency that needs to be manually installed, nor how.

Similarly, it's not obvious in the README that this project requires being on MacOS to work (I first tried it on Linux, where I got an explicit error: PyObjC requires macOS to build error).

I thought putting together a (very basic) install doc could potentially save some trial and error to future users, so, here it is !
Feel free to change or reorganize it as you see fit, and thanks for this project :)

[toadjaune]

For the record : I just tried using this script on a freshly-installed Mac, running MacOS 14.4.1, and those instructions, combined with #27 , were enough to make it work.

No need to manually install a different python version, or anything fancy, just a shebang change and a pip install.