Document the change.

readme-vars.yml

@@ -32,6 +32,7 @@ opt_param_env_vars:
   - { env_var: "CMD_PROTOCOL_USESSL", env_value: "false", desc: "Set to `true` if accessing over https via reverse proxy." }
   - { env_var: "CMD_PORT", env_value: "3000", desc: "If you wish to access hedgedoc at a port different than 80, 443 or 3000, you need to set this to that port (ie. `CMD_PORT=5000`) and change the port mapping accordingly (5000:5000)." }
   - { env_var: "CMD_ALLOW_ORIGIN", env_value: "['localhost']", desc: "Comma-separated list of allowed hostnames"}
+  - { env_var: "CMD_DB_DIALECT", env_value: "", desc: "This variable allows using a database besides sqlite (if DB_HOST not set up) and mysql." }
 param_usage_include_vols: true
 param_volumes:
   - { vol_path: "/config", vol_host_path: "/path/to/{{ project_name }}/config", desc: "Persistent config files" }