Adds deprecation notice for php55 wordpress (GoogleCloudPlatform#844)

Author: bshaffer

appengine/php55/wordpress/README.md

@@ -1,4 +1,8 @@
-# A helper command for running WordPress on Google Cloud Platform
+**NOTE**: The `php55` environment for App Engine uses an EOL version 
+of PHP. Please refer to the [php72 environment](../../php72/wordpress)
+for the current version.
+
+# Running WordPress on App Engine Standard for PHP 5.5
 
 This is a small command line tool for downloading and configuring
 WordPress for Google Cloud Platform. The script allows you to create a