Minor contribution to software publication: To coauthor or not to coauthor?
Imagine the following scenario from the point of view of the main authors. They have been working on a piece of software of which they're proud, and they're writing a paper about it. They were happy with contributions from all kinds of others. It costs them literally nothing to add those others as co-authors to the paper. The resulting goodwill might strengthen the collaborative ties with those others: maybe the contributors might contribute more later. But even if they don't, and this is where the collaboration ends, the main authors have lost nothing by adding more co-authors.

Wolfgang's comment may be spot on: they may be trying to convince you to be part of a larger, longer collaboration. If you would like to be part of that, and if you are confident about the quality of the paper as a whole, I would accept. After all, they are happy to have you in the author list.