Add a general ReadMe to the home directory

[MoffKalast]

I haven't gotten around to writing the thing yet, but in we should provide users of our image a quick rundown of what's where and why so they can get up to speed quickly.

Some things we should include:

• the startup service is in /usr/sbin/magni-base and how to enable logclean in it
• the sonar board leds can be redefined in /boot/config.txt and in the pifi config
• basic pifi info that expands on the offhand mention in the terminal window
• gpio should be accessed via pigpio daemon
• an example of a working ~/.ros/camera_info/extrinsics_demo.yaml