Updates HACKING.rst in training-guides/labs

Adds details for using the osbash scripts
in labs section for development purpose.

Change-Id: I20e05da467a3ea63b15ac4d9f500364bb0e22779
Partial-bug: #1362088

labs/HACKING.rst

@@ -45,6 +45,43 @@ currently all written in bash, the examples to follow are this project and
 DevStack bash style guidelines can be found at the bottom of:
 https://github.com/openstack-dev/devstack/blob/master/HACKING.rst
 
+Structure
+---------
+
+**autostart**
+
+osbash/wbatch copy shell scripts (\*.sh) into this directory to have them
+automatically executed (and removed) upon boot.
+
+**config**
+
+Contains the configuration files for all the scripts. The setup can be customized here.
+
+**img**
+
+By default osbash will put into this directory its base disk images
+(base-\*-<distro>.vdi), the VM export images (oslabs-<distro>.ova),
+and all installation ISO images it may download.
+
+**lib**
+
+This directory contains bash libraries used by scripts.
+
+**log**
+
+Contains the log files written (and removed) by osbash/wbatch and
+the scripts running within the VMs.
+
+**scripts**
+
+All scripts in this directory run within the VMs.
+
+**wbatch**
+
+Files in this directory are Windows batch files generated by osbash to
+configure host-only networks, produce a base disk, and build OpenStack
+training-labs VMs as configured when osbash created them.
+
 Testing
 -------
 
@@ -80,6 +117,11 @@ Reviewing
 Learn how to review (or what to expect when having your patches reviewed) here:
 - https://wiki.openstack.org/wiki/GerritWorkflow
 
+TODO
+----
+Etherpad:
+- https://etherpad.openstack.org/p/training-guides-developing-topics
+
 Anything not covered here
 -------------------------