Add part about the less files

This adds documentation about the `less` files.
It should be relative to the less file. 

Sent from my iPhone

> On Sep 11, 2014, at 9:39 PM, Steven Vachon <notifications@github.com> wrote:
> 
> In docs/pages/migrating.md:
> 
> > +      }
> > +    };
> > +
> > +## Paths in the less files
> > +
> > +Steal compiles the `less` files in a slightly different way than the legacy version which affects `@import`-ing and image urls
> > +
> > +In the legacy Steal, you `@import`-ed stuff relative from the location of the `.less` file:
> > +
> > +    @import '../../../styles/variables.less';
> > +
> > +In the new Steal `@import` path are relative to the steal root folder - folder that contains the `stealconfig.js` file:
> > +
> > +    @import 'styles/variables.less';
> > +
> > +The opposite is the case for the image urls in your `.less` files:
> I take it back, I like it better when it's relative to the app root, heh. Much easier to access bower_components/ without parenting (../). To access the current dir, ./ currently works fine.
> 
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