Add the error message the user will run into

Author: nealwu

official/mnist/README.md

@@ -12,7 +12,7 @@ APIs.
 ## Setup
 
 To begin, you'll simply need the latest version of TensorFlow installed.
-First make sure you've [added the models folder to your Python path](/official/#running-the-models).
+First make sure you've [added the models folder to your Python path](/official/#running-the-models); otherwise you may encounter an error like `ImportError: No module named official.mnist`.
 
 Then to train the model, run the following:
 

official/resnet/README.md

@@ -18,7 +18,7 @@ Please proceed according to which dataset you would like to train/evaluate on:
 ### Setup
 
 You simply need to have the latest version of TensorFlow installed.
-First make sure you've [added the models folder to your Python path](/official/#running-the-models).
+First make sure you've [added the models folder to your Python path](/official/#running-the-models); otherwise you may encounter an error like `ImportError: No module named official.resnet`.
 
 Then download and extract the CIFAR-10 data from Alex's website, specifying the location with the `--data_dir` flag. Run the following:
 

official/wide_deep/README.md

@@ -15,7 +15,7 @@ The input function for the `Estimator` uses `tf.contrib.data.TextLineDataset`, w
 The `Estimator` and `Dataset` APIs are both highly encouraged for fast development and efficient training.
 
 ## Running the code
-First make sure you've [added the models folder to your Python path](/official/#running-the-models).
+First make sure you've [added the models folder to your Python path](/official/#running-the-models); otherwise you may encounter an error like `ImportError: No module named official.wide_deep`.
 
 ### Setup
 The [Census Income Data Set](https://archive.ics.uci.edu/ml/datasets/Census+Income) that this sample uses for training is hosted by the [UC Irvine Machine Learning Repository](https://archive.ics.uci.edu/ml/datasets/). We have provided a script that downloads and cleans the necessary files.