README update for CPU Arch

Author: MaxBed4d

README.md

@@ -17,3 +17,16 @@ This collection includes content for interacting with OpenStack clouds.
 - os_projects
 - os_ratings
 - os_volumes
+
+### Changing platform architecture in os_images
+
+The `Generate diskimage-builder images` job in the ``os_images`` role can build an 
+image for a chosen CPU architecture by defining the ``cpu_arch`` variable before 
+calling the ``os_images`` role. This is an [example](https://github.com/stackhpc/kayobe/blob/18ca11b47af42ce10507516c2f9e34f447d5e39a/ansible/overcloud-host-image-build.yml#L21_) of a playbook calling the ``os_image`` role.
+
+The ``cpu_arch`` variable will also be set as an image property within the `Upload 
+cloud tenant images` task in the ``upload.yml`` task.
+
+If left unset the ``cpu_arch`` variable will default to `x86_64` for ``upload.yml``.
+However, if not set, the `Generate diskimage-builder images` job will takes the 
+disk image builder's default.