Adding some mentions of Stemmer to the Lemmatizer readme

[danyaljj]

Quoting from Dan:

"I wonder if it makes sense for us to add some words about stemming in our Lemmatizer documentation; at this point if people search for a stemmer they don’t get to it, and end up using porter stemmer from NLTK. What I meant here is that we should just include some text in the Lemmatizer documentation, so that people that are looking for a Stemmer will also get to this page, where we will write that lemmateizer is better than a stemmer."