docs(installing/configure-dns): updated xip example and added alternative method

Corrected information about xip.io and added in alternative method for setting up DNS with AWS ELBs.
I think this is just a special case with load balancers. Any other hostname works right out of the box. for example:

```
><> dig deis.bacongobbler.xip.io

; <<>> DiG 9.8.3-P1 <<>> deis.bacongobbler.xip.io
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 19433
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;deis.bacongobbler.xip.io.	IN	A

;; ANSWER SECTION:
deis.bacongobbler.xip.io. 599	IN	A	173.45.254.247
```