Router error on ttl > 0

Nobody has claimed this yet.

I instantiate my client like this:

```
client.portMapping({
public: publicPort,
private: privatePort,
ttl: 0
}, function(err) {
// Will be called once finished
console.log("done", err);
});
```

If ttl is greater than 0 i get this status code 500 and this response from the router api:

``` json
{
"@": {
"xmlns:s": "http://schemas.xmlsoap.org/soap/envelope/",
"s:encodingStyle": "http://schemas.xmlsoap.org/soap/encoding/"
},
"s:Body": {
"s:Fault": {
"faultcode": "s:Client",
"faultstring": "UPnPError",
"detail": {
"UPnPError": {
"@": {
"xmlns": "urn:schemas-upnp-org:control-1-0"
},
"errorCode": "725",
"errorDescription": "OnlyPermanentLeasesSupported"
}
}
}
}
}
```

I found the solution of setting ttl to 0 in this issue https://github.com/zerotier/ZeroTierOne/issues/242
This could be probably added to the README.md file.

Start with README.md and the client.portMapping usage shown in the issue. Document that setting ttl to 0 resolves the router response indicating that only permanent leases are supported, and verify the example clearly communicates this behavior.

Written by the indexing model from the issue text.