Add draft of 'how we built batch changes'

Created by: mrnugget

This is a first draft meaning that I still want to polish it and rephrase certain things. Oh and also add the last two paragraphs. The idea and structure, though, are what I had in mind.

Now that I've written this draft I'm not 100% sure whether this is interesting to readers of the Sourcegraph blog (on my personal blog I don't really have to care about such considerations).

Opinions? Thoughts?