Add documentation

[jhartwell]

Currently there is little to no documentation in the code base, which is not good.

[whossname]

The readme could also use some clarification. The things I noticed were:

1. It would read better if "Getting Plsm" was before "Running Plsm"
2. Make it clearer that generating the config file will append your existing config file instead of deleting it.
3. Give an example for the "destination" field. I have no idea where the models should actually go when generated.

[whossname]

Thanks for this tool btw. I just figured out the "destination" thing and generated the models.

[jhartwell]

@whossname, thanks for the feedback. I will update the readme this week

[jhartwell]

README has been updated. There will be more documentation added along the way as well