Logging configuration only valid on Linux hosts

[euantorano]

It should be documented that the logging configuration as follows only works for Linux based hosts:

    logging:
      driver: syslog
      options:
        tag: "{{.DaemonName}}(image={{.ImageName}};name={{.Name}};id={{.ID}})"

On hosts such as macOS where syslog may not be available show errors such as the following for every service:

ERROR: for dockercompose_mariadb_1  Cannot start service mariadb: failed to initialize logging driver: Unix syslog delivery error

ERROR: for mariadb  Cannot start service mariadb: failed to initialize logging driver: Unix syslog delivery error

ERROR: for memcached  Cannot start service memcached: failed to initialize logging driver: Unix syslog delivery error