PAM changes for supporting duo 2FA solution

Multi-Factor Authentication (MFA) for sshd+PAM
https://duo.com/product/multi-factor-authentication-mfa

Related-Prod: PROD-24422
Change-Id: Iddec5a5e99e7db5d7f173ad939d3fd0cac1cd22b
diff --git a/README.rst b/README.rst
index 127ac3d..79b8b18 100644
--- a/README.rst
+++ b/README.rst
@@ -2175,6 +2175,31 @@
                 shadow: (&(&(objectClass=person)(uidNumber=*))(unixHomeDirectory=*))
                 group:  (&(objectClass=group)(gidNumber=*))
 
+PAM duo 2FA integration
+
+.. code-block:: yaml
+
+    parameters:
+      linux:
+        system:
+          auth:
+            enabled: true
+            duo:
+              enabled: true
+              duo_host: localhost
+              duo_ikey: DUO-INTEGRATION-KEY
+              duo_skey: DUO-SECRET-KEY
+
+duo package version may be specified (optional)
+
+.. code-block:: yaml
+
+      linux:
+        system:
+          package:
+            duo-unix:
+              version: 1.10.1-0
+
 Disabled multipath (the default setup):
 
 .. code-block:: yaml