README.md: Add link to IndieGoGo
README.md: Add link to IndieGoGo

Update description of RabbitMax Flex Raspberry Pi
HAT and add a link to the IndieGoGo campaign.

Signed-off-by: Leon Anavi <be92910ae7bb6f58bb9b7c6f845bc30640119592@anavi.org>

AnaviTech/anavi-examples,AnaviTech/anavi-examples