Refine README.md

[zhangyoufu]

• add --name ipv6nat, avoid random container name
• add read-only bind mount for /lib/modules, so that users don't have to read troubleshooting section
• use "unless-stopped" restart policy, do not start ipv6nat automatically if user stopped it explicitly
• add description about how to load ip6_tables kernel module outside container, for systemd-based distro
• unify docker command line argument style, use --key value instead of --key=value

[bephinix]

@robbertkl Looks good to me!