Update template/README

Add a link to Doxygen documentation about the template, and replace
outdated information about the build system with a link to a more
up-to-date description in the Doxygen documentation.

The links are to the nightly Doxygen build for now and for simplicity,
but in principle could be replaced by links to deployed documentation
for a release once that happens.

Change-Id: I0e9085aa77ab9e473405a8d3619d92fbb4ae255c