message: add mongo_db_plugin descripition
diff: @@ -158,6 +158,7 @@ Note: if you want to use the mongo db plugin, you have to enable it in your `dat
  ```
  # pull images
  docker pull eosio/eos:latest
  +docker pull mongo:latest
  # create volume
  docker volume create --name=nodeos-data-volume
  docker volume create --name=keosd-data-volume
  @@ -173,3 +174,7 @@ docker-compose -f docker-compose-dawn3.0.yaml down
  ```
  The `blocks` data are stored under `--data-dir` by default, and the wallet files are stored under `--wallet-dir` by default, of course you can change these as you want.
  +
  +### About MongoDB Plugin
  +
  +Currently, the mongodb plugin is disabled in `config.ini` by default, you have to change it manually in `config.ini` or you can mount a `config.ini` file to `/opt/eosio/bin/data-dir/config.ini` in the docker-compose file.
  \ No newline at end of file
  