README: add link to angstrom-manifest
README: add link to angstrom-manifest

Signed-off-by: Stefan Agner <eb0877843acc39f8ef6f7269937dee931c372d23@agner.ch>
Signed-off-by: Koen Kooi <42e3d96910f922bdd7dd1805fe3f8aff634f391d@dominion.thruhere.net>

Angstrom-distribution/meta-angstrom,Angstrom-distribution/meta-angstrom,Angstrom-distribution/meta-angstrom,Angstrom-distribution/meta-angstrom