Add step 1 instructions for MaskGAN. (#6917)

* Add step 1 instructions for MaskGAN.

research/maskgan/README.md

@@ -14,8 +14,10 @@ tested. Pretraining may not work correctly.
 
 For training on PTB:
 
-1. Pretrain a LM on PTB and store the checkpoint in `/tmp/pretrain-lm/`.
-Instructions WIP.
+1. Follow instructions here ([Tensorflow RNN Language Model Tutorial](https://www.tensorflow.org/tutorials/sequences/recurrent)) to train a language model on PTB dataset.
+Copy PTB data downloaded from the above tensorflow RNN tutorial to folder "/tmp/ptb". It should contain following three files: ptb.train.txt, ptb.test.txt, ptb.valid.txt
+Make folder /tmp/pretrain-lm and copy checkpoints from above Tensorflow RNN tutorial under this folder.
+
 
 2. Run MaskGAN in MLE pretraining mode. If step 1 was not run, set
 `language_model_ckpt_dir` to empty.