Add ability to configure secure apt mirror

PROD-31387

Change-Id: Idb1dda21694316e4b47b7058954a286fd1386764
diff --git a/README.rst b/README.rst
index 3832eac..a14ae15 100644
--- a/README.rst
+++ b/README.rst
@@ -1135,6 +1135,47 @@
           http:  http://proxy.host.local:3142
           https: https://proxy.host.local:3143
 
+
+Add secured apt repository:
+
+.. code-block:: yaml
+
+    linux:
+      system:
+        ...
+        repo:
+          test:
+            secure: true
+            url: example.org/ubuntu
+            arch: deb
+            protocol: http
+            user: foo
+            password: bar
+            distribution: stable
+            component: main
+
+Add multiply secured apt repositories with same credentials:
+
+.. code-block:: yaml
+
+    linux:
+      system:
+        ...
+        common_repo_secured:
+          arch: deb
+          protocol: http
+          user: foo
+          password: bar
+          distribution: stable
+          component: main
+        repo:
+          test1:
+            secure: true
+            url: example1.org/ubuntu
+          test2:
+            secure: true
+            url: example2.org/ubuntu
+
 Remove all repositories:
 
 .. code-block:: yaml