Thanks for your interest in Common Form! All of the source code behind Common Form is written in JavaScript, managed with Git, licensed under Apache-2.0, developed on GitHub, and distributed via npm. Please have a look around and feel free to ask questions. Contributions of all kinds are welcome.
If you would like to contribute code, please fork the GitHub repository listed in package.json within the package, also available via npm show $PACKAGE_NAME repository.url, then start a new branch off master. Something like git checkout -b $YOUR_BRANCH master should do the trick.
You're also welcome to open an issue on GitHub to discuss the flaw you'd like to fix or the functionality you'd like to add. If you see a branch on GitHub other than master with recent commits, you may want to check whether the package is being rewritten before you dive in.
The easiest way to propose your changes is to open a pull request on GitHub. At least your final commit, but ideally all of your commits, should pass all pre-commit checks, which you can run with npm run pre-commit. Travis CI runs the same checks for code on GitHub with a number of node.js and io.js releases.