Add section on MIPS' Boston board in QEMU user documentation.

Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Aleksandar Rikalo <arikalo@wavecomp.com>
Message-Id: <1551185735-17154-6-git-send-email-aleksandar.markovic@rt-rk.com>