Missing content for nltk

Since nltk needs some data to be downloaded before being used, the notebook `LDA-BBC` will stop, requiring the user to download the needed data.
This can be done with:
`from nltk import download`
`download('punkt')`
`download('averaged_perceptron_tagger')`
`download('wordnet')`
`download('stopwords')`

It may be good to include these lines before running the notebook.