Simple example idea: Create an OpenMM System from a molecule loaded with RDKit

[jchodera]

I was trying to find a simple example illustrating the creation of an OpenMM vacuum System from a molecule loaded with RDKit, and I realized that we may not have such an example around. This might make a good simple example to help folks get started, if it's not already part of the example notebook from last weekend that will be posted.

In general, simple worked examples like this are great resources to point new users towards.