docs: add CONTRIBUTING.md stub
docs: add CONTRIBUTING.md stub

This enables the tests under https://github.com/apiaryio/dredd/pull/1122
to pass. The PR introduces CONTRIBUTING.md and then links it from the
reStructuredText docs, but this would be evaluated as a broken link since
the CONTRIBUTING.md file only exists in the PR and isn't live yet.

apiaryio/dredd,apiaryio/dredd,apiaryio/dredd