Added some text to the callbacks docs. Is not rendering well in tthe Read The...

Added some text to the callbacks docs. Is not rendering well in tthe Read The Docs app, who knows why

docs/callbacks.rst

 Callbacks
 =========
 
+Here are the 4 callbacks available in ``pytorch-widedepp``: ``History`` (which
+is used by default), ``LRHistory``, ``ModelCheckpoint`` and ``EarlyStopping``.
+
 .. autoclass:: pytorch_widedeep.callbacks.History
 	:members:
 	:undoc-members: