Add a quick start example to README

[durkinza]

The README covers detailed steps of each portion of the Firewall documentation, but it fails to provide an easy to view example of what the firewall setup could look like.

Having an example/quickstart section that shows the basic setup of a firewall would be helpful to people that are looking at using this package.

The example only needs to represent the setup code for the Network Firewall, and any immediate dependencies such as the Firewall Policy class. But it doesn't need to show the whole CDK project structure.
The example could represent one of the two "opinionated" approaches that is described in the README, or possibly use the integration test example that is already written.