Add basic DEVELOPERS.md

Author: wfleming

DEVELOPERS.md

@@ -0,0 +1,24 @@
+Information for engine developers & contributors
+
+## Testing local changes
+
+To run the test suite, use `make test`.
+
+Running `make image` will build `codeclimate/codeclimate-rubocop:latest`
+locally. It's a good idea to use a locally-built image to QA engine changes
+against real repos using the Code Climate CLI.
+
+## Releasing a new version
+
+If you are upgrading to a new version of RuboCop, please re-run the doc scraper
+to get any new or updated docs:
+
+```
+docker run \
+  --rm --volume "$PWD:/usr/src/app" --user root --workdir /usr/src/app \
+  codeclimate/codeclimate-rubocop bundle exec rake docs:scrape
+sudo chown -R $(id -u):$(id -g) config
+```
+
+You should add any new cops without docs that fail specs to
+`spec/support/currently_undocumented_cops.txt`.