Document the x-ratelimit-used response header on the "Resources in the REST API" page (github#29242)

Author: timrogers

content/rest/overview/resources-in-the-rest-api.md

@@ -400,13 +400,15 @@ $ curl -I {% data variables.product.api_url_pre %}/users/octocat
 > Date: Mon, 01 Jul 2013 17:27:06 GMT
 > x-ratelimit-limit: 60
 > x-ratelimit-remaining: 56
+> x-ratelimit-used: 4
 > x-ratelimit-reset: 1372700873
 ```
 
 Header Name | Description
 -----------|-----------|
 `x-ratelimit-limit` | The maximum number of requests you're permitted to make per hour.
 `x-ratelimit-remaining` | The number of requests remaining in the current rate limit window.
+`x-ratelimit-used` | The number of requests you've made in the current rate limit window.
 `x-ratelimit-reset` | The time at which the current rate limit window resets in [UTC epoch seconds](http://en.wikipedia.org/wiki/Unix_time).
 
 If you need the time in a different format, any modern programming language can get the job done. For example, if you open up the console on your web browser, you can easily get the reset time as a JavaScript Date object.
@@ -423,6 +425,7 @@ If you exceed the rate limit, an error response returns:
 > Date: Tue, 20 Aug 2013 14:50:41 GMT
 > x-ratelimit-limit: 60
 > x-ratelimit-remaining: 0
+> x-ratelimit-used: 60
 > x-ratelimit-reset: 1377013266
 
 > {
@@ -441,6 +444,7 @@ $ curl -u my_client_id:my_client_secret -I {% data variables.product.api_url_pre
 > Date: Mon, 01 Jul 2013 17:27:06 GMT
 > x-ratelimit-limit: 5000
 > x-ratelimit-remaining: 4966
+> x-ratelimit-used: 34
 > x-ratelimit-reset: 1372700873
 ```