message: add mongodb plugin in readme
diff: @@ -153,12 +153,15 @@ run `docker-compose up`
  We can easliy set up a dawn3.0 local testnet using docker images. Just run the following commands:
  +Note: if you want to use the mongo db plugin, you have to enable it in your `data-dir/config.ini` first.
  +
  ```
  # pull images
  docker pull eosio/eos:dawn3x
  # create volume
  docker volume create --name=nodeos-data-volume
  docker volume create --name=keosd-data-volume
  +docker volume create --name=mongo-data-volume
  # start containers
  docker-compose -f docker-compose-dawn3.0.yaml up -d
  # get chain info
  