docs(rfc): add metagallery template docs

app/docs/template_file.md

@@ -0,0 +1,47 @@
+# Template files
+
+The metagallery template file format is no more than a tarball containing
+the following files:
+
+```txt
+template.tar/
+  |- slots.json
+  |- topview.svg
+  +- scene.glb
+```
+
+The `slots.json` file contains the information about the available slots in
+the scene for the user to drop images/3D models.
+
+```json
+[
+  {
+    "id": "slot1",
+    "type": "2d",
+    "props": {},
+    "v": [
+      [-2.190093, 2.930420, 1.957159],
+      [-2.190091, 1.010420, 1.957159],
+      [-3.686093, 2.930418, 1.957160],
+      [-3.686091, 1.010419, 1.957160]
+    ]
+  },
+  {
+    "id": "slot2",
+    "type": "3d",
+    "props": {
+      "rotating": false,
+      "scale": 1
+    },
+    "v": [
+      [-2.190093, 2.930420, 1.957159]
+    ]
+  },
+]
+```
+
+For `2d` slots, the `v` array should contain the 4 vertices that represent the
+4 corners of the slot frame.
+
+For `3d` slots, the `v` array should contain a single vertex that represents
+the origin position of the 3D model.