Add detailed documentation about how to format .tiff files

[davidmcclure]

When users upload new geotiffs by way of the Omeka interface, the new stores/layers will not work in Neatline unless they are already in spherical mercator projection when the files are uploaded.

If the user has access to Geoserver, she can go into the admin interface and set the "force declared" setting in the layer configuration interface, but we can't expect that users will have this level of access. As far as I can tell from reading through the Geoserver documentation, it's not possible to configure this by way of the API when the files are put to the server.

The documentation needs to be very clear that WMS layers will not work in Neatline unless they are in spherical mercator from the outset. We should also have good instructions about how to set the right EPSG when .tiffs are exported from ArcMap, etc.