* Update backup info in Readme

There are countless arguments on both sides of the Jabber ID vs XMPP address
debate which makes deciding between them a really tough decision.

Pro Jabber ID
* Jabber is easier pronounce
* We have always called it Jabber
* Jabber is more recognizable (This claim can not be backed up by Google Trends)
* Jabber ID has a nicer typography

Pro XMPP address
* People like the term address. People also liked 'Chat address' or
  'Conversations address'. Address is also used in Email address or other
  protocols. Even if people don’t understand the 'XMPP' part of the term they
  might understand the 'address' part and know what is going on.
* While people might have heard of Jabber rather than XMPP; people have heard
  of it in the 00s and associate it with something old. Depending on the
  target audience this is a good thing. And people who value sustainability
  know what XMPP is anyway.
* Jabber is a Cisco product. If we were to succeed in making 'Jabber' cool
  again we don’t want to share that success with Cisco. What has Cisco ever
  done for us? Aside from providing us with a venue for the XSF summit. And
  building nice aqueducts.
* The Cisco owned trademark is a damocles sword. While the XSF technically
  has the right to hand out sublicenses to use the term this can be a lengthy
  process. And automated filter system that for example monitor Google Play
  store descriptions don’t care that the XSF has the rights or that the terms
  of use are more nuanced. They just see a trademark and reject the
  publication. And we all know how impossible it is to speak to an actual
  human at Google.

some modern Androids (namely my Xiaomi Mi A1 after the update to Andorid 9) seem to require that

for example xmpp:foo@bar.tld?join,