message: Add link to SECURITY.md from doc describing model loading.
diff: This document explains how to save and restore
  @{$variables$variables} and models.
  +Important: TensorFlow model files are code. Be careful with untrusted code.
  +See [Using TensorFlow Securely](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/SECURITY.md)
  +for details.
  ## Saving and restoring variables
  