Update readme.txt with GAE only warning

Although this was fixed here https://github.com/GoogleCloudPlatform/appengine-wordpress-plugin/commit/d748edbb6b6a4339078030a2edd75779b1ee6abb#diff-b2bf9bbf2fcc0aab26e7ae461a373b41R450, I feel it is worth adding this warning given it's the most frequent question about the plugin.