Add a note about running wget behind proxy

To avoid mistakes, the wget env variables should be set in
'/root/.wgetrc' since ceph-deploy will run wget as root user on each
node.

Signed-off-by: kfei <kfei@kfei.net>
Set text width to 80 for README

Signed-off-by: kfei <kfei@kfei.net>
Merge pull request #192 from kfei/wget-behind-proxy

Add a note about running wget behind proxy