Add explanation of relation of zoom level to precision

[jywarren]

Ultimately, this library will show a grid on a map, that displays squares of a precision -- so, at one zoom level, it might be latitudes of:

35.01, 35.02, 35.03, 35.04

Like this:

__________________________________________
|72.0,44.3           |72.0,44.4           |
|                    |                    |
|                    |                    |
|                    |                    |
-------------------------------------------
|72.1,44.3           |72.1,44.4           |
|                    |                    |
|                    |                    |
|                    |                    |
-------------------------------------------

Let's use this graphic and also figure out exactly what the precision of latitude/longitude corresponds to each zoom level. Then add this to the README to explain exactly what's going on in a clear way:

(this isn't correct, but to show how we should explain it)

Zoom level 10 will result in a grid with 0.01 spacing
Zoom level 11 -- twice as zoomed in -- will result in a grid with 0.001 spacing
Zoom level 12 will still show 0.001 spacing
Zoom level 13 will start to show 0.0001 spacing

[jywarren]

There's now a lot of explanation that can be copied in in #18