how do I share a package I made?

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There is a tutorial for releasing a ROS package if you want to actually release it as a Debian (which is the most work for you as a developer, but the least work for an end user).

Otherwise, you can simply push the package to a GitHub repo (make sure you specify a License for the code and have a LICENSE file). I'd also recommend making sure that you have a README which details how to use the package and why the package exists. It would also be good to specify in the README which ROS distributions the package should work with.