README: Add a toplevel README
README: Add a toplevel README

Add a toplevel README with some basic information on
layer organization and usage.

Signed-off-by: Moritz Fischer <16c4312f096cb05ec900643fe7a5f22f7761ab4a@ettus.com>

EttusResearch/meta-ettus,EttusResearch/meta-ettus,EttusResearch/meta-ettus,EttusResearch/meta-ettus,EttusResearch/meta-ettus,EttusResearch/meta-ettus