README: fix Waltham links
README: fix Waltham links

Fix the link to Waltham upstream repository, and add a link to the
generated docs.

Signed-off-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>

waltham/waltham,waltham/waltham