docs(autoEncryption): document that autoEncryption is in beta

Fixes NODE-2102

Author: daprahamian

lib/mongo_client.js

@@ -55,7 +55,9 @@ const CloseOperation = require('./operations/close');
  */
 
 /**
- * Configuration options for a automatic client encryption
+ * Configuration options for a automatic client encryption.
+ *
+ * **NOTE**: Support for client side encryption is in beta. Backwards-breaking changes may be made before the final release.
  *
  * @typedef {Object} AutoEncryptionOptions
  * @property {MongoClient} [keyVaultClient] A `MongoClient` used to fetch keys from a key vault