Adds the contributing guide
Merge pull request #28 from nitstorm/contrib_guide

Added the contributing guide.
Added apply_filter() class method to Article to abstract request-based filtering away from the views.
Added apply_filter() method to views that display lists of articles.
Adjusted article preview template to display status of article.
Added apply_filter() to search method.
Added missing import of get_object_or_404 for user registration.
Ensured that only active users are displayed on the home page.
Implemented user profile editor (closes #8).
Added guidelines page for contributing articles to the blog (closes #20).
Fixed article count displayed next to categories.
Fixed sizing of columns on smaller screens for certain pages.
Removed reference to StackTack from Markdown help guide.
Modified base template to allow navbar and footer to be overridden in child templates.
Added image upload form.
Added image upload form template.
Initial (empty) implementation of image upload view.
Removed form-control class from file input widgets.