Update README - subscribe to hooks

Author: braktar

README.md

@@ -139,11 +139,13 @@
  - [Reloading in Rails Applications](#reloading-in-rails-applications)
 - [Performance Monitoring](#performance-monitoring)
  - [Active Support Instrumentation](#active-support-instrumentation)
- - [endpoint_run.grape](#endpoint_rungrape)
- - [endpoint_render.grape](#endpoint_rendergrape)
- - [endpoint_run_filters.grape](#endpoint_run_filtersgrape)
- - [endpoint_run_validators.grape](#endpoint_run_validatorsgrape)
- - [format_response.grape](#format_responsegrape)
+ - [Hook points](#hook-points)
+ - [endpoint_run.grape](#endpoint_rungrape)
+ - [endpoint_render.grape](#endpoint_rendergrape)
+ - [endpoint_run_filters.grape](#endpoint_run_filtersgrape)
+ - [endpoint_run_validators.grape](#endpoint_run_validatorsgrape)
+ - [format_response.grape](#format_responsegrape)
+ - [Subscribe to hooks](#subscribe-to-hooks)
  - [Monitoring Products](#monitoring-products)
 - [Contributing to Grape](#contributing-to-grape)
 - [Security](#security)
@@ -4125,35 +4127,36 @@ See [StackOverflow #3282655](http://stackoverflow.com/questions/3282655/ruby-on-
 
 Grape has built-in support for [ActiveSupport::Notifications](http://api.rubyonrails.org/classes/ActiveSupport/Notifications.html) which provides simple hook points to instrument key parts of your application.
 
-The following are currently supported:
 
-#### endpoint_run.grape
+#### Hook points
+The following hook points are currently supported:
+##### endpoint_run.grape
 
 The main execution of an endpoint, includes filters and rendering.
 
 * *endpoint* - The endpoint instance
 
-#### endpoint_render.grape
+##### endpoint_render.grape
 
 The execution of the main content block of the endpoint.
 
 * *endpoint* - The endpoint instance
 
-#### endpoint_run_filters.grape
+##### endpoint_run_filters.grape
 
 * *endpoint* - The endpoint instance
 * *filters* - The filters being executed
 * *type* - The type of filters (before, before_validation, after_validation, after)
 
-#### endpoint_run_validators.grape
+##### endpoint_run_validators.grape
 
 The execution of validators.
 
 * *endpoint* - The endpoint instance
 * *validators* - The validators being executed
 * *request* - The request being validated
 
-#### format_response.grape
+##### format_response.grape
 
 Serialization or template rendering.
 
@@ -4162,6 +4165,27 @@ Serialization or template rendering.
 
 See the [ActiveSupport::Notifications documentation](http://api.rubyonrails.org/classes/ActiveSupport/Notifications.html) for information on how to subscribe to these events.
 
+#### Subscribe to hooks
+
+Once subscribed to the instrumentation, you can intercept the events reported above.
+
+```
+ActiveSupport::Notifications.subscribe(/<api_path>/) do |name, start, finish, id, payload|
+ # your code to intercept the notification
+end
+```
+
+The request data, the API’s internal data, and the response can be retrieved from the payload.
+
+You can use `payload.fetch(:endpoint)` or directly `payload[:endpoint]`.
+
+The `:endpoint` contains the data currently being processed.
+
+The endpoint provides access to the following attributes:
+`body`, `request`, `params`, `headers`, `cookies`, `response_cookies`
+
+For example, `payload[:endpoint].body` provides the current state of the response
+
 ### Monitoring Products
 
 Grape integrates with following third-party tools: