Added Link to Python Setup Guide [(googleapis#1158)](GoogleCloudPlatform/python-docs-samples#1158)

* Update Readme.rst to add Python setup guide

As requested in b/64770713.

This sample is linked in documentation https://cloud.google.com/bigtable/docs/scaling, and it would make more sense to update the guide here than in the documentation.

* Update README.rst

* Update README.rst

* Update README.rst

* Update README.rst

* Update README.rst

* Update install_deps.tmpl.rst

* Updated readmegen scripts and re-generated related README files

* Fixed the lint error

Author: michaelawyu

samples/metricscaler/README.rst

@@ -31,7 +31,10 @@ credentials for applications.
 Install Dependencies
 ++++++++++++++++++++
 
-#. Install `pip`_ and `virtualenv`_ if you do not already have them.
+#. Install `pip`_ and `virtualenv`_ if you do not already have them. You may want to refer to the `Python Development Environment Setup Guide`_ for Google Cloud Platform for instructions.
+
+ .. _Python Development Environment Setup Guide:
+     https://cloud.google.com/python/setup
 
 #. Create a virtualenv. Samples are compatible with Python 2.7 and 3.4+.