message: docs: update nydusd mount example
  And mount API be adjusted to RESTful style like this:
  `POST /mount` -> Add Mount
  `PUT /mount` -> Update Mount
  `DELETE /mount` -> Remove Mount
diff: @@ -209,13 +209,13 @@ sudo target-fusedev/debug/nydusd \
  --mountpoint /path/to/mountpoint
  ```
  -Then use curl to call the mount api:
  +Then use curl to mount a bootstrap to `/path/to/mountpoint/sub`:
  ``` shell
  curl --unix-socket api.sock \
  - -X PUT "http://localhost/api/v1/mount" -H "accept: */*" \
  + -X POST "http://localhost/api/v1/mount" \
  -H "Content-Type: application/json" \
  - -d "{\"source\":\"<path-to-bootstrap>\",\"fstype\":\"rafs\",\"mountpoint\":\"/path/to/mnt\","config\":\"/path/to/config.json\"}"
  + -d '{"source":"<path-to-bootstrap>","fstype":"rafs","mountpoint":"/sub","config":"/path/to/config.json"}'
  ```
  ### Multiple Pseudo Mounts
  