Update document for client usage

[k-ogawa-1988]

As I see README.md for client configuration, it shows like this:

version: '2'

services:
  client:
    image: balenablocks/multiroom
    command: [ "/usr/bin/snapclient", "--host", "<MULTIROOM_SERVER>" ]  # "<MULTIROOM_SERVER>" is your multiroom server hostname or IP address

But in entry.sh, host parameter should be --host=<MULTIROOM_SERVER>.

Thus, docker command should be like this:

version: '2'

services:
  client:
    image: balenablocks/multiroom
    command: [ "/usr/bin/snapclient", "--host=<MULTIROOM_SERVER>" ]  # "<MULTIROOM_SERVER>" is your multiroom server hostname or IP address

This works well for me.

• Server: using balenaSound
• Client: my custom docker-compose.yml

Thank you.