Use testable example and update README.md.

This commit moves the example to a test file so it integrates nicely with
Go's example tooling.

This allows the example output to be tested as a part of running the
normal Go tests to help ensure it doesn't get out of date with the code.
It is also nice to have the example in one place rather than repeating it
in doc.go and README.md.

Links and information about the example have been included in README.md in
place of the example.