Add graphical description of nanoMIPS instruction pool organization.

Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Aleksandar Rikalo <arikalo@wavecomp.com>
Message-Id: <1551800076-8104-3-git-send-email-aleksandar.markovic@rt-rk.com>