Update README.md to indicate required TensorFlow version.

im2txt/README.md

@@ -119,7 +119,7 @@ approximately 10 times slower.
 First ensure that you have installed the following required packages:
 
 * **Bazel** ([instructions](http://bazel.io/docs/install.html)).
-* **TensorFlow** ([instructions](https://www.tensorflow.org/versions/master/get_started/os_setup.html)).
+* **TensorFlow** r0.12 or greater ([instructions](https://www.tensorflow.org/versions/master/get_started/os_setup.html)).
 * **NumPy** ([instructions](http://www.scipy.org/install.html)).
 * **Natural Language Toolkit (NLTK)**:
     * First install NLTK ([instructions](http://www.nltk.org/install.html)).