Add HUBOT_IMGUR_CLIENTID env variable setup to install instructions.

I've instaled this script following the README instructions but soon I've found
it wasn't working. After taking a quick look to the source code, I've saw the
"Configuration" note.

I think it's better to add this step to the install instructions @ README so
anyone can install it quickly.
Merge pull request #1 from matagus/installation-add-clientid

Add HUBOT_IMGUR_CLIENTID env variable setup to install instructions.