Merge pull request #813 from pnomolos/issue_453_pg_json_equality_note

Add a note about DISTINCT and PostgreSQL json columns.

README.md

@@ -492,6 +492,14 @@ def index
 end
 ```
 
+#### `PG::UndefinedFunction: ERROR: could not identify an equality operator for type json`
+
+If you get the above error while using `distinct: true` that means that
+one of the columns that Ransack is selecting is a `json` column.
+PostgreSQL does not provide comparison operators for the `json` type.  While
+it is possible to work around this, in practice it's much better to convert those
+to `jsonb`, as [recommended by the PostgreSQL documentation](https://www.postgresql.org/docs/9.6/static/datatype-json.html).
+
 ### Authorization (whitelisting/blacklisting)
 
 By default, searching and sorting are authorized on any column of your model