Add example usage

[felberj]

Could you put an example about how to use the remill library? Something like the documentation at https://github.com/trailofbits/remill/blob/master/docs/LIFE_OF_AN_INSTRUCTION.md but with actual c++ code calling it.

I think there would be a big benefit for people that are interested in using the library

[pgoodman]

For now the best example is Lift.cpp from the tests. Besides that I will work toward adding a walkthrough.