document how to port apps from rM1

[raisjn]

For an rM1 app to work on the rM2, they need to do the following:

• install rm2fb server and client libraries
• run rm2fb server (see README.md)
• run the rm1 app with LD_PRELOAD=/path/to/client.so <app>

The rM1 app will now be able to draw to screen.

NOTE: the other two main pieces of compat are the battery location and handling inputs, see Eeems-Org/oxide#48 for discussion of those two.