docs: add missing docs for err.sqlState

Author: dougwilson

Changes.md

@@ -310,6 +310,7 @@ you spot any mistakes.
 ## v2.0.0-alpha9 (2013-08-27)
 
 * Add query to pool to execute queries directly using the pool
+* Add `sqlState` property to `Error` objects #556
 * Pool option to set queue limit
 * Pool sends 'connection' event when it opens a new connection
 * Added stringifyObjects option to treat input as strings rather than objects (#501)

Readme.md

@@ -1168,6 +1168,7 @@ object. Additionally they typically come with two extra properties:
 * `err.sql`: String, contains the full SQL of the failed query. This can be
   useful when using a higher level interface like an ORM that is generating
   the queries.
+* `err.sqlState`: String, contains the five-character SQLSTATE value. Only populated from [MySQL server error][].
 * `err.sqlMessage`: String, contains the message string that provides a
   textual description of the error. Only populated from [MySQL server error][].