Add a statement on install/index.md on what os are supported.

PiperOrigin-RevId: 168032996

tensorflow/docs_src/install/index.md

 # 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: