GitHub is the best channel to give feedback to the developers of Transcrypt or to contribute in general.
Developing Transcrypt would have been impossible without the valuable feedback received from many people, using GitHub, but also via email. Feel free to use either medium to share your experiences, wishes and proposals. This is how sourcemaps and static typechecking became part of Transcrypt!
On the other hand, don't be too disappointed if you're proposal doesn't make it into the distribution. There are many, often contradictory, requirements to consider. But be sure that it will be looked into seriously.
Especially bug reports were indispensible to make Transcrypt reliable. Something wrong? File a GitHub issue so it can be corrected, possibly already in the next commit. Not sure? File it nevertheless, better safe than sorry.
The availability of small, diverse examples enables people to become quickly acquainted with Transcrypt. You're invited to share what you've learned, e.g. using GitHub issues. Such examples may become part of the documentation.
If you think a certain aspect of Transcrypt deserves more attention in the documentation, consider writing a proposed new or improved paragraph about it. The eventual format is restructured text for Sphinx, but plain text contributions are also welcome.
In general tutorials and course material are very useful and will be considered for inclusion in, or linking from the Transcrypt website.
Another way to contribute to Transcrypt is to improve or augment the code. Many people have already contributed fixes and enhancements, mostly as GitHub issues. If you decide to contribute in this way, please keep the design goals in mind. Simple and straight: Transcrypt aims at approaching CPython as closely as possible while remaining lean, fast and open. This leads to the following guidelines:
Do you like Transcrypt? Does it help you to write better software or to have more pleasure in writing it? Talk about it with colleagues, write a blog post about it or give a presentation. Mention it in the credits of your application, star it on GitHub, pose or answer questions about it on StackOverflow. In short: get it on the radar of as many people as possible. Give developers the possibility to choose.