Mention how to populate environment:/ URIs in Sirius example

[agarciadom]

In the example for working with Sirius models from Epsilon through a plain Java program, we should mention how to register the Environment.odesign into the URIConverter.URI_MAP manually (which is needed to provide some standard values, like the default color palette). It's something of this sort:

URIConverter.URI_MAP.put(
  URI.createURI("environment:/viewpoint"),
  URI.createURI(DescriptionPackage.class.getResource("/model/Environment.odesign").toURI().toString())
);