README: find resource(s) for getting lat/long coordinates for boundary boxes

[mwesterfield]

We need easy-to-use websites that can provide lat/long coordinates for locations in San Diego. Preferably, the user should be able to point and click on a map and retrieve coordinates; it would be even better if the user could drag a box on the map and get the four boundary points. These websites should be listed in the "Resource for working with the data" section of the README.md file.

[swingley]

Here's a site that will let you manipulate a box on a map and show you the coordinates for a bounding box: https://boundingbox.klokantech.com/

If you select CSV as the output type you'll get a string like:
-117.699014,32.871363,-117.212055,33.068737

The format for that is minLongitude, minLatitude, maxLongitude, minLatitude which is unfortunate because we need minLatitude:minLongitude,maxLatitude:maxLongitude e.g. '32.696015:-117.174882,32.72834:-117.14311'