Add a small README for the Ansible proxy image role

proxy/ansible/README.md

+Ansible role to set up a generic VM image with a reverse proxy that
+participates in the live.a.o pool. The image should be able to do the
+following automatically and without any intervention, on first boot:
+
+* generate a unique persistent host identifier for the VM
+* start the agent and register with the central registry, so that
+  a DNS entry for the VM is created
+* acquire a valid SSL certificate using Letsencrypt
+* advertise itself as ready-to-serve and start receiving traffic
+
+While on shutdown it should implement the following process:
+
+* advertise status as not-ready-to-serve
+* switch NGINX configuration to serve redirects back to the main
+  server, to avoid breaking streams for clients
+* after some time (currently 5 minutes), actually shut down the VM
+
+The above is implemented via systemd dependencies and simple shell
+scripts.