Fix Stretch Gazebo Rviz config and update README

[hello-binit]

TODO

• List all topics that diff between simulated and real
  • Write a quick guide on using "remap", "group", and "if/unless" evaluation to handle sim/real and other branches in launch files.
• Add image of random_objects.world

[nickswalker]

Thanks for improving the simulator and docs! In addition to noting the different topics (which will be very helpful), it could be useful to suggest how to patch some of the differences via remaps or otherwise. We've been using the following in our Gazebo bringup: https://github.com/hcrlab/hcrl_gazebo/blob/9d06dd7fff59376be9e76410c85b635146d968da/launch/include/common.xml#L21

[hello-binit]

Good call @nickswalker, I'll add some notes on using the remap, eval, etc. tools within launch files.