Extend section on topic modelling

[stefan-mueller]

One of the most frequently asked questions is how to use a dfm for topic models. We cover topic models in one section, but I think we should also show how to convert the dfm object to a stm topic model and how to run a basic operation.

We might also add links to tutorials on topic models, such as:

• Training, evaluating, and interpreting topic models using stm
• Select number of topics

What's your view, @koheiw?

[koheiw]

I am a topic model skeptic, who do not believe that we can conduct solid social science research using (unsupervised) topic models, so not willing to extend the topic model page much. I usually cover those special topics in slides like https://cdn.rawgit.com/koheiw/workshop-PolText2018/master/slides.html

[kbenoit]

I could extend the topic models page, since I also get tons of questions on this, and was planning to integrate STM better soon anyway. (including a PR to the stm package).