Add Stack Overflow document source

[sylvinus]

Dumps seem to be available at https://archive.org/details/stackexchange

[wumpus]

This is a good idea, you can do a much better job indexing Stack Exchange sites from the data dump. As an example, the tags (like "Python", "Ruby" etc) are only sometimes in the question title, but people searching stack overflow frequently put a language in their query. Yeah, you can find the tags in the html somewhere, but it's probably easier to use the data dumps directly.