add caas environment readme

Author: sjpb

environments/.caas/README.md

@@ -1,5 +1,18 @@
 # Caas cluster
 
-Default Azimuth Slurm
+Environment for default Azimuth Slurm. This is not intended to be manually deployed.
 
-See the main README.md in the repo root for an overview and general install instructions.  Any environment-specific instructions should be added here.
+Non-standard things for this environment:
+- There is no activate script.
+- `ansible.cgf` is provided in the repo root, as expected by the caas operator.
+- `ANSIBLE_INVENTORY` is set in the cluster type template, using a path relative to the 
+  runner project directory:
+
+        azimuth_caas_stackhpc_slurm_appliance_template:
+        ...
+        envVars:
+            ANSIBLE_INVENTORY: environments/common/inventory,environments/.caas/inventory
+
+    Ansible then defines `ansible_inventory_sources` which contains absolute paths, and 
+    that is used to derive the `appliances_environment_root` and 
+    `appliances_repository_root`.