.travis.yml: Document how the build matrix use caches

We will set the CACHE_NAME variable to improve the caching
of various jobs using the same characteristics. Document it
first.

See https://docs.travis-ci.com/user/caching/#caches-and-build-matrices



Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>