docs: improve comments and remove unnecessary lines

- Add a comment about creating a session with a POST request to `/api/v1/sessions`
- Remove the line mentioning `yarn cli` command
- Add a comment about using the `_id` field in the response
- Remove the example usage of `yarn cli`

Signed-off-by: wancat <wancat@wancat.cc>

README.md

@@ -40,8 +40,22 @@ yarn app
 
 ## CLI terminal client
 
-Open server first, then
+Open server first, then create a session by `POST /api/v1/sessions`
+
+```bash
+curl -X 'POST' \
+  'http://localhost:3000/api/v1/sessions' \
+  -H 'accept: application/json' \
+  -H 'Content-Type: application/json' \
+  -d '{
+  "quest_id": "helloworld"
+}'
+```
+
+Then use the `_id` field in response
 
 ```
-yarn cli
+yarn cli [_id]
+# example
+yarn cli 64be226a39c0043cd1cdf7c2
 ```