Improved README readablity
Improved README readablity

Added a section about what the theme actually does.
Updated links to guides to point to what will be the urls in the new_guides branch.
Removed section about creating a dummy test app, this belongs in the guide.
Added a section about creating your own extensions, with links to the guide.

quentinuys/spree_fancy,rsempe/spree_fancy,holden/spree_fancy,rsempe/spree_fancy,kewaunited/spree_fancy,samuels410/spree_fancy,bhanu22776/spree_fancy_theme,mvjohn100/spree_fancy,mvjohn100/spree_fancy,rsempe/spree_fancy,spree-contrib/spree_fancy,kewaunited/spree_fancy,holden/spree_fancy,bhanu22776/spree_fancy_theme,spree-contrib/spree_fancy,samuels410/spree_fancy,kewaunited/spree_fancy,quentinuys/spree_fancy,holden/nannakay-template,holden/nannakay-template,spree-contrib/spree_fancy,quentinuys/spree_fancy