engine: updated out-of-date README.branding

Added info to README.branding about some recent additions such as
external_resources.properties and BrandingCascadingResourceServlet.

Change-Id: Idf5b4e3c1f47adefe0ee37f580a7aa6954f2d3d5
Signed-off-by: Greg Sheremeta <gshereme@redhat.com>