Provide documentation for EasyShare customization

Nuxeo EasyShare provides a "hook" for customizing the CSS of the download page (i.e. the public page that is displayed to non-authenticated users). A developer may deploy a file at /nuxeo/css/nuxeo-easyshare-override.css. This file is loaded by the EasyShare download page and can be used to customize the layout.

This is not mentioned anywhere in the documentation, nor the readme for the module. I only found out by looking at SUPNXP-31451

It would be good to explain how the file can be used as well (as opposed to having to dig through the source).