Add foss.training's onion address

[anweshadas]

https://foss.training/ is the official website for Dgplug Summer Training. foss.training has an onion address now.

http://aknfm77troykzirpefkq23pwjmfuq3xnuny3ztrmyhvjti5vzxq2eiqd.onion/

You can verify the address from the website footer.