Update throttle.md to describe --throttle-HTTP

Author: ClareCat

doc/throttle.md

@@ -46,6 +46,14 @@ Note that you may dynamically change both `--max-lag-millis` and the `throttle-c
 
 An example query could be: `--throttle-query="select hour(now()) between 8 and 17"` which implies throttling auto-starts `8:00am` and migration auto-resumes at `18:00pm`.
 
+#### HTTP Throttle
+
+The `--throttle-HTTP` flag allows for throttling via HTTP. Every 100ms `gh-ost` issues a `HEAD` request to the provided URL. If the response status code is not `200` throttling will kick in until a `200` response status code is returned.
+
+If no URL is provided or the URL provided doesn't contain the scheme then the HTTP check will be disabled. For example `--throttle-HTTP="http://1.2.3.4:6789/throttle"` will enable the HTTP check/throttling, but `--throttle-HTTP=1.2.3.4:6789/throttle` will not.
+
+The URL can be queried and updated dynamically via [interactive interface](interactive-commands.md).
+
 #### Manual control
 
 In addition to the above, you are able to take control and throttle the operation any time you like.