Add warning about Fast-RTPS (#387)
Add warning about Fast-RTPS (#387)

issue #365

Also fixed typo "Publishing a messages" > "Publishing a message"

Signed-off-by: maryaB-osr <8d1b748a20efbd0dd05268dc1894c5c52a2851b8@openrobotics.org>
ros2/demos,ros2/demos,ros2/demos,ros2/demos