README: Update docs

Add a pointer to the online signing document and delete some redundant
information.  Update the "embedding" section and add more background info
on what it is used for.  Eliminate references to $BUILD_SECURE which no
longer exists.

Change-Id: I6d971849cc21697de9cf4fd891423f331f083830
Signed-off-by: Kevin Cernekee <cernekee@google.com>