Add note about memcache whitelist (GoogleCloudPlatform#755)

This is just for people who are browsing on
Github and might not be aware (I made this
mistake).

Author: waprin

appengine/flexible/memcache/README.md

@@ -1,3 +1,22 @@
+## Note
+
+This sample demonstrates connecting to existing Memcache servers, or the
+built-in Memcache server.
+
+A managed option for Memcache is RedisLabs:
+
+https://cloud.google.com/appengine/docs/flexible/python/using-redislabs-memcache
+
+You can install and manage a Memcache server on Google Compute Engine. One way
+to do so is to use a Bitnami click-to-deploy:
+
+https://bitnami.com/stack/memcached/cloud/google
+
+Built-in Memcache for Flexible environments is currently in a whitelist-only alpha. To have your project whitelisted,
+see the signup form here:
+
+https://cloud.google.com/appengine/docs/flexible/python/upgrading#memcache_service
+
 ## Running locally
 
 Refer to the [top-level README](../README.md) for instructions on running and deploying.

appengine/flexible/memcache/main.py

@@ -23,6 +23,7 @@
 
 # [START client]
 # Environment variables are defined in app.yaml.
+# Note: USE_GAE_MEMCACHE is in whitelist-only alpha. See README.md
 if os.environ.get('USE_GAE_MEMCACHE'):
     MEMCACHE_SERVER = ':'.join([
         os.environ.get('GAE_MEMCACHE_HOST', 'localhost'),