Add linux.system.apt state

Change-Id: I1e190f1f5b34a049335efbecffad3dddf7c41644
diff --git a/README.rst b/README.rst
index 1cbc18c..cafb41c 100644
--- a/README.rst
+++ b/README.rst
@@ -626,6 +626,21 @@
       system:
         purge_repos: true
 
+Setup custom apt config options:
+
+.. code-block:: yaml
+
+    linux:
+      system:
+        apt:
+          config:
+            compression-workaround:
+              "Acquire::CompressionTypes::Order": "gz"
+            docker-clean:
+              "DPkg::Post-Invoke":
+                - "rm -f /var/cache/apt/archives/*.deb /var/cache/apt/archives/partial/*.deb /var/cache/apt/*.bin || true"
+              "APT::Update::Post-Invoke":
+                - "rm -f /var/cache/apt/archives/*.deb /var/cache/apt/archives/partial/*.deb /var/cache/apt/*.bin || true"
 
 RC
 ~~