Fix index description

Mention that participants woudld be contributing to Sugar.

Temporary link to creative commons website. Until we define
the proper license to be used. Including its version.

Remove how-to-test description. To be moved to a different
web page.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Add steps to test the background images

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>