Add postgres official documentation

free-programming-books.md

@@ -1502,6 +1502,7 @@ See also [TeX](#tex)
 
 
 ### PostgreSQL
+* [Postgres Official Documentation](http://www.postgresql.org/docs/)
 * [Postgres Succinctly](http://www.syncfusion.com/resources/techportal/ebooks/postgres) (PDF, Kindle) *(Just fill the fields with any values)*
 * [Practical PostgreSQL](http://www.commandprompt.com/ppbook/)