docs(portforwarding): Explain how to use custom port forwarding command

[lavalleeale]

See qdm12/gluetun#2399 for non-documentation discussion

[qdm12]

Suggested change

	- Can be set via `VPN_PORT_FORWARDING_UP_COMMAND=command`
	- Doesn't understand shell specific syntax such as `&&`, and one should use `/bin/sh -c "my shell syntax"` to do so if they want (bash isn't installed by default, only sh)
	- Will have the string `{{PORTS}}` replaced by a comma separated list of the ports that have been forwarded (Example: `VPN_PORT_FORWARDING_UP_COMMAND=/script.sh {{PORTS}}`)
	A command can be set with `VPN_PORT_FORWARDING_UP_COMMAND`. For example `VPN_PORT_FORWARDING_UP_COMMAND=/bin/sh -c "echo {{PORTS}}"`.
	Notes:
	- The special string `{{PORTS}}` is replaced by a comma separated list of the ports that have been forwarded. For example `/bin/sh -c "echo {{PORTS}}"` would become `/bin/sh -c "echo 5678,9876"`
	- shell specific syntax such as `&&` is not understood in the command, and one should use `/bin/sh -c "my shell syntax"` to do so if they want.
	- one can bind mount a shell script in Gluetun and execute it with for example `VPN_PORT_FORWARDING_UP_COMMAND=/bin/sh -c /gluetun/myscript.sh` - 💁 feel free to propose a pull request to add commonly used shell scripts for port forwarding!
	- the output of the command is written to the port forwarding logger within Gluetun