Document ISAR_TURTLEBOT_TURTLEBOT_MAP

[luteberget]

Describe the improvement you would like to see
I am running isar-turtlebot gazebo simulation setup with the "house" model which is available by default from the turtlebot vendor.
Needed to set the following environment variable:
ISAR_TURTLEBOT_TURTLEBOT_MAP=turtleworld

This was slightly tricky to find out. Unless I missed something, this could be added to the readme file or something so that it becomes easier to get the most straight-forward isar/turtlebot simulation setup.

How will this change existing functionality?
Documentation only.

How will this improvement affect the current Threat Model?
Leave blank for maintainers to fill out if you are uncertain about this.

[luteberget]

Note that ISAR crashes on startup if you do not add this, because it's missing the file klab_turtlebot.json. Should this file be in package_data?

[aeshub]

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