Add documentation to README.md

[aronwc]

• Please keep track of each stage of processing and document it in README.md.

That is, enumerate step-by-step how to go from the raw data to the output of your analysis.

[aronwc]

First draft looks good. We discussed:

• adding way to load db schema in one command
• fix markdown layout
• show sample output (scatterplot and cross-validation scores)