Rework README.md and introduce a new 'doc' folder which includes
additional documentation covering a variety of topics related to
CHRE and nanoapp development.

Bug: 160347248
Test: n/a, markdown changes only
Merged-In: Ie1c66b07ca7c2abc2b439d23a7ec1e56da530d3b
Change-Id: Ie1c66b07ca7c2abc2b439d23a7ec1e56da530d3b