added API section to readme.

README.md

@@ -34,6 +34,65 @@ The short code has the following configuration options.
 
 `[wplibcalhours location="Parnassus Library" num_weeks=1]` - Prints the opening hours for the "Parnassus Library" for the next week (starting today).
 
+## API
+
+This plugin also exposes opening hours via a read-only, public API endpoint. 
+
+Requesting the path `/wp-admin/admin-ajax.php?action=wplibcalhours` on your site will yield
+a JSON-formatted payload comprised of dates and opening hours, keyed by their location names.
+
+These lists are sorted in chronological order and contain day-of-the-week, date and the day's opening hours information.
+
+Each list starts with the current date.
+
+#### Example payload
+
+Three locations, the current date is January 3rd.
+
+```json
+{
+  "Parnassus Library": [
+    {
+      "day": "Wed",
+      "date": "Jan 3",
+      "text": "7:45am - 10pm"
+    },
+    {
+      "day": "Thu",
+      "date": "Jan 4",
+      "text": "7:45am - 10pm"
+    }
+    //...
+  ],
+  "Mission Bay Library": [
+    {
+      "day": "Wed",
+      "date": "Jan 3",
+      "text": "9am - 6pm"
+    },
+    {
+      "day": "Thu",
+      "date": "Jan 4",
+      "text": "9am - 6pm"
+    }
+    // ...
+  ],
+  "Mission Bay Hub": [
+    {
+      "day": "Wed",
+      "date": "Jan 3",
+      "text": "24 hours"
+    },
+    {
+      "day": "Thu",
+      "date": "Jan 4",
+      "text": "24 hours"
+    }
+    // ...
+  ]
+}
+```
+
 ## Styling
 
 Please see this [Wiki page](https://github.com/ucsf-ckm/wplibcalhours/wiki/Styling-The-Output) for ideas on how to customize the styles of this plugin's generated markup.