meta-aspeed: enhance README.md

Add words to the layer README.  The existing README is somewhat terse.
Expand it some, adopting some of the conventions from other well-known
OE BSP layers (used meta-freescale as a template).

Drop references to specific products.  This reduces maintainence burden.

Added layer dependencies.  meta-aspeed depends on oe-core only.

Add contributing guidelines and style guide.

(From meta-aspeed rev: 5e3e6fb63cbc700214fb3354363d93a5fa4246df)

Change-Id: Ib4ec545c7fe5069893090516232fc2d9c666c2c8
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>