Update intro in Docs

[bannanc]

I had lumped all of the documentation updates into one issue when there were really two.

1. Example, API, and general writing feedback which was a part of issue Doc cleanup #25
2. More extensive update including a better introduction and explanation of the problem chemper solves (this issue)

Below is a list of things to think about provided by @jmaat way back in June.

• make sure to start with the really broad motivation
• Very clear definition of atom typing and chemical perception, why this is so important to MD/compchem
  • point to other potential uses.
• define/pick a better word for molecular subgraph

• add more justification to chemper intro, bash atom types, etc
• make a backstory/history/OFF_connection/big picture/ section
  • define combinatorial problem and smirks space
  • move smarty/smirky background here

From David (originally in issue #25)

• Intro: "Then will automatically generate reasonable SMIRKS patterns matching clustering of molecular subgraphs," missing subject. Also add Jessica under contributors as she's helping with docs at least?

README

• more explicit why we are interested in this at the top