README.rst: update instructions to ease the access to the built ISO

When installer ISO images are built using the "tiler" container, they are generated under "/var/tmp/tiler/" inside the container. To allow direct access to this location on the host, we mount the directory "/var/tmp/tiler/" on the host into the "tiler" container.

This patch also specifies the ISO filename format so that the reader may find an easier time identifying the ISO under the output directory.

Signed-off-by: William Lyu William.Lyu@windriver.com