Add AJAX recipe to documentation

[nhunzaker]

Easy enough. cc @ltk

I haven't covered cancellation or progress. These are special cases that I want to give their own recipes. Thoughts?

[coveralls]

Coverage remained the same at 100.0% when pulling 56dc8db on ajax-recipe into 030491b on master.

[coveralls]

Coverage remained the same at 100.0% when pulling e1dcce9 on ajax-recipe into 030491b on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 5d89c90 on ajax-recipe into 030491b on master.

[coveralls]

Coverage remained the same at 100.0% when pulling cf78b02 on ajax-recipe into 030491b on master.

[ltk]

noice 👍

[coveralls]

Coverage remained the same at 100.0% when pulling 55d25c4 on ajax-recipe into 95fafd7 on master.