add plug to sentry docs

Author: mitchellhenke

docs/index.rst

@@ -118,6 +118,7 @@ Want more?  Have a look at the full documentation for more information.
 
    usage
    config
+   plug
 
 Resources:
 

docs/plug.rst

@@ -0,0 +1,25 @@
+Sentry.Plug
+=============
+
+Sentry.Plug provides basic funcitonality to handle Plug.ErrorHandler.
+
+To capture errors, simply put the following in your router:
+
+.. code-block:: elixir
+
+  use Sentry.Plug
+
+Optional settings
+------------------
+
+.. describe:: body_scrubber
+
+  The function to call before sending the body of the request to Sentry.  It will default to ``Sentry.Plug.default_body_scrubber/1``, which will remove sensitive parameters like "password", "passwd", "secret", or any values resembling a credit card.
+
+.. describe:: header_scrubber
+
+  The function to call before sending the headers of the request to Sentry.  It will default to ``Sentry.Plug.default_header_scrubber/1``, which will remove "Authorization" and "Authentication" headers.
+
+.. describe:: request_id_header
+
+  If you're using Phoenix, Plug.RequestId, or another method to set a request ID response header, and would like to include that information with errors reported by Sentry.Plug, the `:request_id_header` option allows you to set which header key Sentry should check.  It will default to "x-request-id", which Plug.RequestId (and therefore Phoenix) also default to.