Wm notes 1.15.0

Pinging @mglukhovsky (and @larkost if he wants to check my checking on pull requests and issues).
We need to add a section about the new datum serialization format -- that is, you won't see full performance improvements for some operations (relating to #1915) until you re-insert data into RethinkDB (e.g. r.table().insert(r.table()) + rename table -- though there may be a better way to do this).

This is a big deal, because the massive performance improvements of #1915 won't be seen unless the user does this.

@mlucy, @timmaxw, @coffeemug, @danielmewes -- as far as I know, you're all involved in #1915 in different ways. Could you propose an approach for users?