As an MDN writer, I’m kinda biased, but here goes
There are different schools of thought on this. Many people (myself included) think that documentation is an essential part of any project (open source or otherwise). You need documentation if you want a decent number of users or contributors, and if you want your project to be usable/maintainable/installable.
Some people seem to regard documentation as an afterthought, but without it I believe your project’s community will be limited to the elitists that built the thing in the first place, or were smart enough to figure it out in spite of the lack of docs.
One excuse for minimal or no documentation is lack of expertise in writing documentation — it is a distinct skill, and one that engineers are often not so good at. I am happy to accept this to a degree — bad documentation can be worse than no documentation. But you should still be willing to accept that your project needs documentation, and try to find someone to help.
There is a lively community dedicated to documentation: http://www.writethedocs.org/. This is another place that might be useful for you to ask questions at.