Contributing new features
If you create a new custom command or something else which would be useful for other Drupal installations, please consider forking the Drupal Console project on GitHub. Then just create an issue for a "feature request", put your work in a new Git branch on your fork, publish your branch and add a pull request on GitHub (including your issue ID in the PR title). The Drupal Console maintainers are very happy to accept useful contributions and usually do so quite promptly.
Getting support with becoming a contributor
If you want to contribute to Drupal Console and have any difficulty getting oriented to the process, you can find Instant Messaging support on Gitter IM.
Standard GitHub Workflow
If you haven't yet contributed to a project on GitHub, or are not sure you remember the workflow, you might first want to read the documentation about pull requests. You may also wish to download the GitHub application (Mac | Windows, which simplifies the workflow a bit and provides a nice GUI for your contributions).
This documentation is written in “Github-flavored Markdown”, which is rendered on Github, directly, as HTML. If you are already familiar with Markdown, GFL only adds a few useful tweaks, otherwise you may want to read Github’s Markdown Basics primer.