Add design section to CONTRIBUTING.md
Add design section to CONTRIBUTING.md

ember-cli/ember-twiddle,knownasilya/ember-twiddle,Gaurav0/ember-twiddle,pangratz/ember-twiddle,knownasilya/ember-twiddle,ember-cli/ember-twiddle,Gaurav0/ember-twiddle,ember-cli/ember-twiddle,pangratz/ember-twiddle,pangratz/ember-twiddle,Gaurav0/ember-twiddle,knownasilya/ember-twiddle