Document setting JSON_API_UNIFORM_EXCEPTIONS

Fixes #326

Extracted from #437

Author: sliverc

docs/usage.md

@@ -237,6 +237,13 @@ class MyViewset(ModelViewSet):
 
 ```
 
+### Exception handling
+
+For the `exception_handler` class, if the optional `JSON_API_UNIFORM_EXCEPTIONS` is set to True,
+all exceptions will respond with the JSON API [error format](http://jsonapi.org/format/#error-objects).
+
+When `JSON_API_UNIFORM_EXCEPTIONS` is False (the default), non-JSON API views will respond
+with the normal DRF error format.
 
 ### Performance Testing