Add CMake instructions to apple-runtime.md (#5533)

Summary:
Took me a bit to figure out how to make cmake work; I had it installed in homebrew but it wasn't working.

Add instructions for installing cmake globally in a way that Xcode can find it.

Pull Request resolved: https://github.com/pytorch/executorch/pull/5533

Reviewed By: shoumikhin

Differential Revision: D63153558

Pulled By: dbort

fbshipit-source-id: b7df7c0b9723c6e60cafea67a39dd326276a6148