Update kernels readme (#4274)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4274

Update kernels readme with Cmake instructions. Major changes are:
1. Instructions to generate the operator function signature with cmake instead of buck.
2. Instructions to build and test the operator with cmake.
3. Update links from buck to cmake, internal to oss

Move existing buck instructions to fb/ directory.

Reviewed By: larryliu0820, kirklandsign

Differential Revision: D59784834

fbshipit-source-id: 4a73c43b57ce3d87fa8e162b2e5417ba068f0c45