Updated Readme.md

Updated Readme.md with documentation for the 'getUserFromClient' model function

Author: lucknerjb

Readme.md

@@ -231,6 +231,21 @@ The spec does not actually require that you revoke the old token - hence this is
      - Must contain the following keys:
          - *string|number* **id**
 
+### Required for `client_credentials` grant type
+
+#### getUserFromClient (clientId, clientSecret, callback)
+- *string* **clientId**
+- *string* **clientSecret**
+- *function* **callback (error, user)**
+ - *mixed* **error**
+     - Truthy to indicate an error
+ - *object* **user**
+     - The user retrieved from storage or falsey to indicate an invalid user
+     - Saved in `req.user`
+     - Must contain the following keys:
+         - *string|number* **id**
+
+
 ### Optional
 
 #### generateToken (type, callback)