MAINTAINERS: sctp: move Neil to CREDITS

Neil moved away from SCTP related duties.
Move him to CREDITS then and while at it, update SCTP
project website.

Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>