Update README re. what it looks like when Stamen has poor tile data

- ggmap_3.0.0.900
- ggplot2_3.1.0
- R 3.5.2

---

I was wondering why a map I was building would sometimes get errors in some `maptype`s, and return a completely blank plot (i.e. a completely white background except for points) with other map types. Eventually I found that it's because Stamen does not have good tiles for that region.

It would be good if the readme could show users what happens in these cases, and what it looks like.

I will submit a PR for this later.

```
dat <-
data.frame(
lat = c(-17.46915, -17.46915, -17.46915, -17.46915, -17.46963,
-17.46963, -17.46963, -17.46963, -17.46896, -17.46896,
-17.46896, -17.46896, -17.470533, -17.470533, -17.470533, -17.470533,
-17.4703, -17.4703, -17.4703, -17.4703, -17.469583, -17.469583,
-17.469583, -17.469583, -17.469938, -17.469938, -17.471302,
-17.470467, -17.470467, -17.470467, -17.470832),
lon = c(140.79945, 140.79945, 140.79945, 140.79945, 140.79967,
140.79967, 140.79967, 140.79967, 140.79927, 140.79927,
140.79927, 140.79927, 140.796717, 140.796717, 140.796717, 140.796717,
140.796683, 140.796683, 140.796683, 140.796683, 140.796367,
140.796367, 140.796367, 140.796367, 140.793741, 140.793741,
140.794474, 140.795435, 140.795435, 140.795435, 140.795601)
)

qmplot(lon, lat, data = dat, maptype = "terrain", zoom = 18)

## Using zoom = 18...
## Source : http://tile.stamen.com/terrain/18/233594/143994.png
## Not Found (HTTP 404). Failed to aquire tile /terrain/18/233594/143994.png.
## Source : http://tile.stamen.com/terrain/18/233595/143994.png
## Not Found (HTTP 404). Failed to aquire tile /terrain/18/233595/143994.png.
## Source : http://tile.stamen.com/terrain/18/233596/143994.png
## Not Found (HTTP 404). Failed to aquire tile /terrain/18/233596/143994.png.
## Source : http://tile.stamen.com/terrain/18/233597/143994.png
## Not Found (HTTP 404). Failed to aquire tile /terrain/18/233597/143994.png.
## Source : http://tile.stamen.com/terrain/18/233598/143994.png
## Not Found (HTTP 404). Failed to aquire tile /terrain/18/233598/143994.png.
## Source : http://tile.stamen.com/terrain/18/233599/143994.png
## Not Found (HTTP 404). Failed to aquire tile /terrain/18/233599/143994.png.
## Source : http://tile.stamen.com/terrain/18/233594/143995.png
## Not Found (HTTP 404). Failed to aquire tile /terrain/18/233594/143995.png.
## Source : http://tile.stamen.com/terrain/18/233595/143995.png
## Not Found (HTTP 404). Failed to aquire tile /terrain/18/233595/143995.png.
## Source : http://tile.stamen.com/terrain/18/233596/143995.png
## Not Found (HTTP 404). Failed to aquire tile /terrain/18/233596/143995.png.
## Source : http://tile.stamen.com/terrain/18/233597/143995.png
## Not Found (HTTP 404). Failed to aquire tile /terrain/18/233597/143995.png.
## Source : http://tile.stamen.com/terrain/18/233598/143995.png
## Not Found (HTTP 404). Failed to aquire tile /terrain/18/233598/143995.png.
## Source : http://tile.stamen.com/terrain/18/233599/143995.png
## Not Found (HTTP 404). Failed to aquire tile /terrain/18/233599/143995.png.
## Source : http://tile.stamen.com/terrain/18/233594/143996.png
## Not Found (HTTP 404). Failed to aquire tile /terrain/18/233594/143996.png.
## Source : http://tile.stamen.com/terrain/18/233595/143996.png
## Not Found (HTTP 404). Failed to aquire tile /terrain/18/233595/143996.png.
## Source : http://tile.stamen.com/terrain/18/233596/143996.png
## Not Found (HTTP 404). Failed to aquire tile /terrain/18/233596/143996.png.
## Source : http://tile.stamen.com/terrain/18/233597/143996.png
## Not Found (HTTP 404). Failed to aquire tile /terrain/18/233597/143996.png.
## Source : http://tile.stamen.com/terrain/18/233598/143996.png
## Not Found (HTTP 404). Failed to aquire tile /terrain/18/233598/143996.png.
## Source : http://tile.stamen.com/terrain/18/233599/143996.png
## Not Found (HTTP 404). Failed to aquire tile /terrain/18/233599/143996.png.

qmplot(lon, lat, data = dat, maptype = "terrain-background", zoom = 18)

## A white plot with points superimposed.

qmplot(lon, lat, data = dat, maptype = "watercolor", zoom = 18)

## A plot that shows the sea and the land

```