Add client_id to get token endpoint

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

docs/spec/auth/token.md

@@ -111,6 +111,16 @@ Defines getting a bearer and refresh token using the token endpoint.
         subject with different scopes. The refresh token does not have an
         expiration and should be considered completely opaque to the client.
     </dd>
+    <dt>
+        <code>client_id</code>
+    </dt>
+    <dd>
+        String identifying the client. This client_id does not need
+        to be registered with the authorization server but should be set to a
+        meaningful value in order to allow auditing keys created by unregistered
+        clients. Accepted syntax is defined in
+        [RFC6749 Appendix A.1](https://tools.ietf.org/html/rfc6749#appendix-A.1).
+    </dd>
     <dt>
         <code>scope</code>
     </dt>