writing-bots.md: Add fixture JSON skeleton.

templates/zerver/api/writing-bots.md

@@ -464,7 +464,22 @@ following helper method:
         # self.assert_bot_response(...)
 
 `mock_http_conversation(fixture_name)` patches `requests.get` and returns the data specified
-in the file `fixtures/<fixture_name>.py`. For an example, check out the [giphy bot](
+in the file `fixtures/<fixture_name>.json`. Use the following JSON code as a skeleton for new
+fixtures:
+```
+{
+  "request": {
+    "api_url": "http://api.example.com/",
+    "params": {
+    }
+  },
+  "response": {
+  },
+  "response-headers": {
+  }
+}
+```
+For an example, check out the [giphy bot](
 https://github.com/zulip/python-zulip-api/tree/master/zulip_bots/zulip_bots/bots/giphy).
 
 *Tip: You can use [requestb.in](http://requestb.in) or a similar tool to capture payloads from the