Add Diceware reference for truly random passphrase

Humans are terrible at entropy and terrible at choosing random passwords. They are equally terrible at remembering the kind of random character strings recommended for passwords.

Most people think they are making a unique passphrase, even choosing phrases from books and so on, and these have been constantly been hacked, usually within minutes of funding wallets such created. 

Diceware solves this problem by using real words, like Electrum Wallet seeds, which have both real entropy and are easy to remember.

I think it's really important to link to this reference lest more people get burned.