README: Add basic Angstrom instructions
README: Add basic Angstrom instructions

Provide very basic Angstrom build instructions and a link to the online
documentation.

Signed-off-by: Darren Hart <38f9eaa30910c40b06f5a60020b1222f2a953801@linux.intel.com>
Acked-by: Koen Kooi <42e3d96910f922bdd7dd1805fe3f8aff634f391d@dominion.thruhere.net>

koenkooi/meta-minnow,koenkooi/meta-minnow