Update installation instructions.

README.rst

@@ -13,11 +13,17 @@ Automatically asks for MFA token key to retrieve temporary credentials.
 Installation and Usage
 ----------------------
 
+Install the plugin:
+
 .. code-block:: sh
 
-   python setup.py install
-   aws configure set plugins.credentials awscli_plugin_credential_mfa
+   pip install awscli-plugin-credential-mfa
+
+Enable the plugin:
 
+.. code-block:: sh
+
+   aws configure set plugins.credentials awscli_plugin_credential_mfa
 
 Assuming your IAM user has `MFA enabled <https://docs.aws.amazon.com/IAM/latest/UserGuide/tutorial_users-self-manage-mfa-and-creds.html>`_ and credentials set up, you also need to set ``mfa_serial`` and
 ``source_profile`` in the configuration. Below are instructions for profile ``default``, adjust as