Update readme [(#1124)](GoogleCloudPlatform/python-docs-samples#1124)

* update readme

* keep entity sentiment in v1beta2 as well

* update readme

Author: dizcology

samples/snippets/README.rst

@@ -76,7 +76,7 @@ To run this sample:
     $ python snippets.py
 
     usage: snippets.py [-h]
-                       {sentiment-text,sentiment-file,entities-text,entities-file,syntax-text,syntax-file}
+                       {sentiment-entities-text,sentiment-entities-file,sentiment-text,sentiment-file,entities-text,entities-file,syntax-text,syntax-file}
                        ...
     
     This application demonstrates how to perform basic operations with the
@@ -86,7 +86,12 @@ To run this sample:
     https://cloud.google.com/natural-language/docs.
     
     positional arguments:
-      {sentiment-text,sentiment-file,entities-text,entities-file,syntax-text,syntax-file}
+      {sentiment-entities-text,sentiment-entities-file,sentiment-text,sentiment-file,entities-text,entities-file,syntax-text,syntax-file}
+        sentiment-entities-text
+                            Detects entity sentiment in the provided text.
+        sentiment-entities-file
+                            Detects entity sentiment in a Google Cloud Storage
+                            file.
         sentiment-text      Detects sentiment in the text.
         sentiment-file      Detects sentiment in the file located in Google Cloud
                             Storage.