removed checkpoint save_function option

docs/Trainer/Checkpointing.md

@@ -0,0 +1,18 @@
+Lightning can automate saving and loading checkpoints.
+
+---
+### Model saving
+To enable checkpointing, define the checkpoint callback
+
+``` {.python}
+from pytorch_lightning.utils.pt_callbacks import ModelCheckpoint
+
+checkpoint = ModelCheckpoint(
+    filepath='/path/to/store/weights.ckpt',
+    save_function=None,
+    save_best_only=not hparams.keep_all_checkpoints,
+    verbose=True,
+    monitor=hparams.model_save_monitor_value,
+    mode=hparams.model_save_monitor_mode
+)
+```