Add first draft of the request convention.

REQUEST_FORMAT.md

+# Request format
+All micro services for the related items system adhere to the following conventions.
+
+The micro service is queried via the POST endpoint: `/search`.
+The request contains the POST params `term` and `context`.
+
+The micro service returns a json object containing the response `type`
+and type-dependent data. e.g.
+```
+{
+    "type": "image",
+    "path": "/path/to/image"
+}
+```
+
+```
+{
+    "type": "miss"
+}
+```