Add port info to readme about docker-jdownloader

[jiaz83]

short explanation what the direct connection mode does.
client(app,webinterface,tool...)<-....->JDownloader connections happens either

1.) client<-apiserver->JDownloader
in this(default,fallback) mode both(control- and data-) connections are using the api server.
Advantage: no need to forward ports/dyndns
Disadvantage: lower bandwidth and higher latency

2.) client<->JDownloader
in this (direct connection) mode, control connections are still using the api server while data connections are directly connecting to the running JDownloader instance without any relay server
Advantage: much higher bandwidth and reduced latency
Disadvantage: user might have to manually enable/allow port forwarding from LAN and/or WAN IP to JDownloader instance
On connection issues, the client will automatically fallback to 1.) and try to re-establish a direct connection again.

by default direct connection mode is set to LAN, so only clients from LAN can connect directly.
see https://support.jdownloader.org/Knowledgebase/Article/View/33/0/myjdownloader-advanced-settings
default port is 3129