need environment variable set for simplecov to work. Documenting requ…

…irement to help someone out in the future.

CONTRIBUTING.md

@@ -35,7 +35,7 @@ Ideally, a bug report should include a pull request with failing specs.
 3. Add specs for your unimplemented feature or bug fix.
 4. Run `bundle exec rake spec`. If your specs pass, return to step 3.
 5. Implement your feature or bug fix.
-6. Run `bundle exec rake spec`. If your specs fail, return to step 5.
+6. Run `COVERAGE=true bundle exec rake spec`. If your specs fail, return to step 5.
 7. Run `open coverage/index.html`. If your changes are not completely covered
    by your tests, return to step 3.
 8. Add, commit, and push your changes.