Should I write a paper on open-source software I've built?
I had faced this exact situations a few years ago. I had developed an open source project, which went pretty popular and started being used by many corporations across the globe. Since, I was looking for strengthening my presence in academia, I planned to write a paper on it and sent to a decent journal. The result was rejection with a plethora of useful comments and suggestions. It took me almost an year to publish that paper in a reputable conference. 

So, the lessons learnt were (and possibly applicable to you as well)


The expectations of research tracks is not only that your thing just works, but you need to justify how and why it works
There should be results (mostly analytical), to demonstrate that your technique works and hence can be used by other people referring/reading it.
You need to answer why your work is important and what it contributes to the field
Moreover, most conferences have or accept papers from industrial or application tracks, where you can describe your work in an implementation centric way (possibly giving you some leverage from nuances of research papers as mentioned above)