docs: update Contributing Link in Readme
docs: update Contributing Link in Readme

Fix a 404 Error because the path of the CONTRIBUTING.md was changed and
not updated in the README.md

vue-styleguidist/vue-styleguidist,vue-styleguidist/vue-styleguidist,vue-styleguidist/vue-styleguidist,vue-styleguidist/vue-styleguidist