Fill in the empty readmes

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

affine/playbook.yml

@@ -1,5 +1,5 @@
 #################################################
-# Setup Overleaf
+# Setup Affine
 #################################################
 ---
 - hosts: monoco

affine/readme.md

@@ -0,0 +1,19 @@
+# Affine
+
+Runs [Affine](https://affine.pro) on `monoco` as Docker containers, with its
+state in the shared Postgres and Valkey units.
+
+## Variables
+
+- `host`: public hostname, `wiki.tanshu.com`.
+- `revision`: image tag, `stable` by default.
+- `db_password`: password for the `affine` database.
+
+Set them in [vars/default.yml](vars/default.yml).
+
+## Run
+
+    ansible-playbook playbook.yml
+
+The `affine-migration` role is commented out. Enable it for the first run and
+after a major version upgrade.

overleaf/readme.md

@@ -0,0 +1,18 @@
+# Overleaf
+
+Runs the [Overleaf Community Edition](https://github.com/overleaf/overleaf)
+`sharelatex` image on the hosts in `all`, with its state in the shared Mongo
+and Redis units.
+
+## Variables
+
+- `http_host`: public hostname, `latex.tanshu.com`.
+- `host_port`: host port, `3199`.
+- `sharelatex_image`: image name.
+- `mongo_container` and `redis_container`: the data services it expects.
+
+Set them in [vars/default.yml](vars/default.yml).
+
+## Run
+
+    ansible-playbook playbook.yml

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