Fix link to dev blog in contributing docs
Fix link to dev blog in contributing docs

Closes #287

mysociety/popit,openstate/popit,openstate/popit,Sinar/popit,Sinar/popit,mysociety/popit,openstate/popit,mysociety/popit,mysociety/popit,Sinar/popit,Sinar/popit,openstate/popit,mysociety/popit