Add to documentation: How to plot unfilled polygons onto another map

It was really hard for me to find out that the facecolor of a plot has to be 'none' instead of None to plot an unfilled polygon. I needed it to plot two tables into one figure (see below).

We could add an information to the API-documentation:

**style_kwds : dict
    Color options to be passed on to the actual plot function, such as edgecolor, facecolor,
-   linewidth, markersize, alpha.
+   linewidth, markersize, alpha. Use 'facecolor='none' to get an unfilled polygon.

Or we could add an example to the “Making maps” section.

What do you think?

ax = df1.plot(column='windzone', edgecolor='#888888', linewidth=0.5, cmap=my_blue)
df2.plot(edgecolor='black', alpha=1, facecolor='none', ax=ax, linewidth=1.5)
plt.show()

[ardieorden]

@uvchik @jorisvandenbossche Hi, I’d like to take on this issue!

[ljwolf]

The fact that facecolor='none' is so in the weeds is why I prefer doing world.boundary.plot(). I find that to be both more explicit and clear, but I’m not sure if there’s a performance penalty for it.