Need documentation on exporting data via TopoJSON

[alukach]

We need documentation in README.md for how a user can export data as TopoJSON from Dropchop.

[alukach]

@jczaplew Interested in adding this?

[jczaplew]

Yep! I can take care of it, along with #240 which is very similar

[mapsam]

@jczaplew you should link to your blog post as well, it was a fascinating deep dive.