Add doc.

plugins/inputs/webhooks/README.md

@@ -15,6 +15,7 @@ $ sudo service telegraf start
 
 ## Available webhooks
 
+- [Filestack](filestack/)
 - [Github](github/)
 - [Mandrill](mandrill/)
 - [Rollbar](rollbar/)

plugins/inputs/webhooks/filestack/README.md

@@ -0,0 +1,15 @@
+# Filestack webhook
+
+You should configure your Filestack's Webhooks to point at the `webhooks` service. To do this go to `filestack.com/`, select your app and click `Credentials > Webhooks`. In the resulting page, set the `URL` to `http://<my_ip>:1619/filestack`, and click on `Add`.
+
+## Events
+
+See the [webhook doc](https://www.filestack.com/docs/webhooks).
+
+All events for logs the original timestamp, the event name and the id.
+
+**Tags:**
+* 'action' = `event.action` string
+
+**Fields:**
+* 'id' = `event.id` string