Add README to `example` directory

[joshuadavidthomas]

For someone clicking around the GitHub UI trying to get a sense of the library, may be useful to have a README within the example directory that copies the installation instructions from the base README, along with some more details regarding what all features the examples are showing off.

We could then just reference the README within the documentation, and have it also be there as well.

[jefftriplett]

It might be worth adding a note about examples/navigation.py in the "Open your browser to http://localhost:8000 to see the examples in action." section of the examples README.