Add model instructions to README #146

Ahh! This makes a lot more sense now. We should probably add a comment somewhere in 2a_model.R (maybe above p2a_med_obs_data and p2a_well_obs_data) that reminds a user that x_vars needs to be updated in this config file if any variables are added/omitted.

Originally posted by @lekoenig in #139 (comment)

It'd be useful if the README contained instructions for running the model, modifying the model (including adding new variables), and creating a new set of model experiments.