Contributions are always welcome, no matter how large or small. Before contributing, please read the code of conduct.
To start developing on babylon you only need to install its dependencies:
npm install
After this step you can now start and run the tests:
npm test
If you are making changes to babylon which make it necessary to also change things in babel you will want to link both repositories together. This can be done by doing the following (assuming you have both babel and babylon already checked out):
cd babylon/
npm link
npm run build
cd ../babel/
make bootstrap
npm link babylon
cd packages/babel-core/
npm link babylon
cd ../../packages/babel-template/
npm link babylon
cd ../../packages/babel-traverse/
npm link babylon
cd ../../packages/babel-generator/
npm link babylon
cd ../..
make build
make test
From now on babel will use your local checkout of babylon for its tests.