added explanatory sentence about map dots.

* Needed to add some more explanation for users to understand what they're seeing on the map. Sentence is _"Blue dots indicate things that have occurred within the past week. Click in to see more details."_
* Not ready to merge.
* @livienyin When you get a chance, will you take a look at this? Right now the sentence is placed under the map, but I think it could look more elegant if it was in a small transparent overlay in the upper left hand corner of the map. Thoughts?