Move kernels/portable/README.md to kernels/

Summary: IIUC, this README is describing the entire kernels/ subtree, not kernels/portable. Move it and leave a breadcrumb in its old location.

Reviewed By: cccclai

Differential Revision: D56494181

fbshipit-source-id: aa3be6fd9dc8b5201ca12596f3a2b6398a31b3a3

Author: swolchok