cramfs: rehabilitate it

Update documentation, pointer to latest tools, appoint myself as
maintainer. Given it's been unloved for so long, I don't expect anyone
will protest.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Tested-by: Chris Brandt <chris.brandt@renesas.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>