docs: document "invalid token" error

closes #768

Author: jedwards1211

README.md

@@ -297,7 +297,8 @@ Error object:
 
 * name: 'JsonWebTokenError'
 * message:
-  * 'jwt malformed'
+  * 'invalid token' - the header or payload could not be parsed
+  * 'jwt malformed' - the token does not have three components (delimited by a `.`)
   * 'jwt signature is required'
   * 'invalid signature'
   * 'jwt audience invalid. expected: [OPTIONS AUDIENCE]'