Explain why it's not possible to add d3_views integration for raw data.

libraries/d3.sankey/d3.sankey.libraries.info

@@ -5,3 +5,16 @@ version = 0.1
 dependencies[] = d3-plugins-sankey
 dependencies[] = d3.chart
 dependencies[] = d3.chart.sankey
+; Unfortunately, raw data integration with d3_views is not possible because:
+; - the structure of the nodes[] and links[] arrays doesn't really fit with any
+;   of the `__data_type`s defined in that module - we would need a something
+;   similar to a '2dnav' mapper, except that we would need to lock down how many
+;   keys can be used (1 or 2 for nodes[]; 2 or 3 for links[]) and the keys
+;   themselves ('name', and 'id' for nodes[]; 'value', 'source', and 'target'
+;   for links[]);
+; - we cannot write a replacement mapper for Sankey charts ourselves because
+;   neither d3_get_library_info_handler() nor \d3_views_plugin_style_d3::init()
+;   provides us with a way to override the Views plugin for a specific type of
+;   chart; and;
+; - if we override the original d3_views plugins, we will conflict with other
+;   modules that need to do the same thing.