Fill in the empty readmes

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

overleaf/readme.md

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+# 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