Added an instruction for installing WiFi drivers

This is to be executed if WiFi does not work by default.

I assumed it is of higher priority than the "theme instruction", as the
latter is of purely cosmetic nature. Feel free to rearrange the entries,
if you feel different about it.
Removed a somewhat redundant word
Moved the WiFi driver instruction to the very end

I assume, that I may not be the only one expecting the most recent official build on elementaryos.org, instead of some sourceforge repository. Thus I personally think this could still be of value for some people, until the link in the journal article gets updated.
But maybe that's just me...

In the end it's completely up to you, whether or not to include it.
I just wanted to make one last proposal ;-)
Pointed out, that the problem may occur on Beta 1
Merge pull request #19 from mank319/master

Added an instruction for installing WiFi drivers