server: add README

server/README.md

@@ -0,0 +1,18 @@
+# SpaceAPI endpoints
+
+The API is currently available under https://spaceapi.it-syndikat.org/api/. The following endpoints
+exist:
+- `GET status.php`: the current space status in [SpaceAPI](https://spaceapi.io/) format, registered
+  in the [SpaceAPI directory](https://github.com/SpaceApi/directory)
+- `GET status-s.php`: returns `true` if space is open, `false` if closed. Called by discourse theme
+  and serendipity.
+- `GET update.php`: updates the current open status, called by the wood box with two fat buttons
+  ("2fb").
+- `POST ping.php`: requests a "spaceping". Called by the
+  [matrix bot](https://git.it-syndikat.org/IT-Syndikat/its-matrix-bot).
+- `GET ping-get.php`: returns `true` if a ping request is pending, then resets the request. Called
+  by 2fb.
+
+All endpoints except for `status[-s].php` require an API key to be passed via the `apikey=` GET
+parameter. These API keys are defined in `config.inc.php`, see `config.inc.php.example` for an
+example.