Add design notes for inventory persistence features

doc/design.txt

@@ -21,3 +21,24 @@ Ansible was identified as being a suitable replacement for the
 builtin SSH logic and for Chef, it made even more sense to continue
 using YAML for the file format because users could use the same
 format for configuring Bang and for authoring Ansible playbooks.
+
+
+Inventory Persistence
+---------------------
+It is often useful to have access to the inventory that was used
+during a particular Bang run.  Bang already provides the inventory
+and the host variables to Ansible directly as Python objects when
+executed as ``bang``, and as a JSON object output to stdout when
+executed as an Ansible inventory plugin (i.e. ``bang --list``).  It
+should also provide the following features:
+
+    - Store inventory and host variables in a user-specified file.
+    - Create ``latest-inventory`` symlink to inventory file after
+      each Bang run.
+    - Allow configuration of inventory output format (i.e. YAML or
+      JSON).  Even though the Ansible inventory plugin API uses JSON
+      as the serialization format, Bang's default inventory output
+      should be YAML for symmetry since Bang's input format is YAML
+      as well.
+    - Allow configuration of above via command-line arguments,
+      ~/.bangrc, or even ansible.cfg.