color.rst is a little long already, and we wish to get more
documentation that may be hard to integrate in color.rst (making it even
longer, not quite fitting with the topic, or needing features like math
markup). Therefore start splitting it up and collect a ToC straight in
the README.

This patch moves the design goals as-is into a new file. The design
goals are considered mature and unlikely to change, unlike some other
parts.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>