modified stories readme to include sample preprocessing code to split stories to 1k tokens

Example usage:

```

# Preprocess the dataset:

# Note that the dataset release is the full data, but the paper models the first 1000 words of each story

# Here is some example code that can trim the dataset to the first 1000 words of each story

$ python

$ data = ["train", "test", "valid"]

$ for name in data:

$   with open(name + ".wp_target") as f:

$     stories = f.readlines()

$   stories = [" ".join(i.split()[0:1000]) for i in stories]

$   with open(name + ".wp_target", "w") as o:

$     for line in stories:

$       o.write(line.strip() + "\n")

# Binarize the dataset:

$ TEXT=examples/stories/writingPrompts

$ python preprocess.py --source-lang wp_source --target-lang wp_target \