Add examples of WMS configurations

[bferguso]

Should add some different examples of using a Web Mapping Service as a source for map layers. May want to consider adding some/all of the following:
Ways to add sources:

• Django Admin Pages (description in README.MD)
• Django Fixture
• Django Migration

Different Source Types:

• Raster
• Vector

[mradamcox]

Here's an example of using a Django migration to automatically load a map layer stored in a fixture: https://github.com/legiongis/fpan/blob/master/fpan/migrations/0015_load_slr_layers.py

We need to keep in mind that Arches can also load map layers automatically during package installation as well, so we'll need to make sure these two strategies are clearly documented.