PMG code: comments and brief documentation

[ColSmith76]

Add more detailed comments to the PMG code, and create a brief markdown document describing the steps, Includes example inputs and outputs under different settings/for a couple of distinct markets (later to be incorporated in the overall user guide)

[ColSmith76]

Outline of document:

1. Purpose
2. Mathematical structure of the games
3. Parameters and their effects
4. R code implementation
5. Testing results
6. Sensitivity of the PMG