Update README.md to clarify models require Nightly and how to update Python path in colab (#6098)

* Update README.md

1. Clarify instructions to avoid friction. https://github.com/tensorflow/tensorflow/issues/24136
2. If trying to run in Colab experienced issue entering Python path. https://medium.com/@cwcgchello/how-to-set-pythonpath-in-colab-bf8c239c7c29

* Update README.md

official/README.md

@@ -14,11 +14,21 @@ If you are on a version of TensorFlow earlier than 1.4, please [update your inst
 ## Requirements
 Please follow the below steps before running models in this repo:
 
-1. Add the top-level ***/models*** folder to the Python path with the command:
+
+1. TensorFlow [nightly binaries](https://github.com/tensorflow/tensorflow#installation)
+
+2. Add the top-level ***/models*** folder to the Python path with the command:
    ```
    export PYTHONPATH="$PYTHONPATH:/path/to/models"
    ```
-2. Install dependencies:
+   
+   Using Colab:
+   ```
+   import os
+   os.environ['PYTHONPATH'] += ":/path/to/models"
+   ```
+
+3. Install dependencies:
    ```
    pip3 install --user -r official/requirements.txt
    ```