GCP DSN example for flexible environment (go-sql-driver#993)

* Added GCP connection string example for flexible environment.

* Added one more GCP connection string example for flexible environment.

* Unified GCP connection strings examples for 2nd gen and flexible env.

Author: cn0047

AUTHORS

@@ -81,6 +81,7 @@ Steven Hartland <steven.hartland at multiplay.co.uk>
 Thomas Wodarek <wodarekwebpage at gmail.com>
 Tim Ruffles <timruffles at gmail.com>
 Tom Jenkinson <tom at tjenkinson.me>
+Vladimir Kovpak <cn007b at gmail.com>
 Xiangyu Hu <xiangyu.hu at outlook.com>
 Xiaobing Jiang <s7v7nislands at gmail.com>
 Xiuming Chen <cc at cxm.cc>

README.md

@@ -403,7 +403,7 @@ user@cloudsql(project-id:instance-name)/dbname
 
 Google Cloud SQL on App Engine (Second Generation MySQL Server):
 ```
-user@cloudsql(project-id:regionname:instance-name)/dbname
+user:password@unix(/cloudsql/project-id:region-name:instance-name)/dbname
 ```
 
 TCP using default port (3306) on localhost: