README: Document how to build
README: Document how to build

The build process for risu is a bit more complex now we have
a configure script, so describe it in the README.

Signed-off-by: Peter Maydell <313afa09ce093b19ae1dd74f57c1ebd94508584c@linaro.org>

stsquad/risu,vivier/risu,stsquad/risu,stsquad/risu,vivier/risu