Add API section to readme

README.md

@@ -72,6 +72,20 @@ expired(headers)
 // false
 ```
 
+### API
+
+#### expired(headers)
+
+Returns a boolean relating to whether the resource has expired or not. `true` means it's expired, `false` means it's fresh.
+
+#### expired.in(headers)
+
+Returns the amount of milliseconds until the resource will expire. If the resource has already expired it will return a negative integer.
+
+#### expired.on(headers)
+
+Returns a JavaScript `Date` object for the date the resource will expire.
+
 ## License
 
 MIT © Luke Childs