README: add jekyll installation instruction for Debian

On Sat, Jul 05, 2014 at 10:22:20PM -0700, Matt Davis wrote:
> > @@ -264,8 +264,15 @@ and the information about your bootcamp.
> >  
> >  **Note:**
> >  you will need to install Jekyll 1.0.3 or later in order to preview things locally.
> > -If you have Ruby installed on your computer,
> > -this *should* be as simple as:
> > +
> > +If you are using Debian or Ubuntu or a derivative, this is as simple as:
> 
> Since very few people are likely to be in this boat I suggest putting these instructions down after the ones for Mac.

After looking at the order again, I still think that this is the proper place:
above the generic instructions with 'gem install ...'. If someone is using a Mac,
they'll know that the instruction for Debian do not apply to them. If
someone is using a Linux distribution, installing using gem is possible but an
inferior option, so it's better if they use the distribution-provided packages
rather than follow generic installation instructions.

BTW., I have now pushed an updated commit which adds a few more packages (make,
pandoc, ...) to the list of packages to install. AFAICT, github silently substituted
the commit, and the pull request shows this updated commit with your comment,
but just to make sure: the new one is 38200fedb15a4bf40f265352591354fa95f6d4c9.