message: Add a statement on install/index.md on what os are supported.
diff: # Installing TensorFlow
  +We've built and tested TensorFlow on the following 64-bit laptop/desktop
  +operating systems:
  + * MacOS X 10.11 (El Capitan) or later.
  + * Ubuntu 14.04 or later
  + * Windows 7 or later.
  +Although you might be able to install TensorFlow on other laptop or desktop
  +systems, we only support (and only fix issues in) the preceding configurations.
  +
  The following guides explain how to install a version of TensorFlow
  that enables you to write applications in Python:
  