Fill in the empty readmes

Affine, Overleaf, and the registry had empty readmes, which the layout check now rejects.

registry/readme.md

@@ -0,0 +1,18 @@
+# Docker registry
+
+Runs the [distribution registry](https://distribution.github.io/distribution/)
+v3 on `clair`, behind the shared Caddy unit.
+
+## Variables
+
+- `host`: public hostname, `registry.tanshu.com`.
+- `username` and `password`: basic-auth credentials for pushes and pulls.
+- `docker_image`: registry image, `registry:3`.
+- `tracing_enabled`: set to `true` to enable the registry's tracing endpoint.
+
+Set them in [vars/default.yml](vars/default.yml). The password is a real
+credential, so move it to ansible-vault before this playbook is shared.
+
+## Run
+
+    ansible-playbook playbook.yml