However, if I ran into this incidentally (not posted to HN for review), I would never use this. I'd see the lack of a github link on the top right, and assume it's a some kind of startup making some prototype hosted tool.
The only way to find out this is open-source is to click "about," read to the third paragraph, see github mentioned, click through the link, and click on the license.
My major piece of feedback is to add a github link to the icons at the top-right of the page. A .org might also be nicer than a .com.
To people asking why this and not a graphical tool?
To me, the overhead to moving a graphical tool is very large:
1) I like being able to manage files on github and be able to use common tooling.
2) If someone (including myself, two years later) needs to install Vizio, Illustrator, or whatever other tool to edit my diagrams, pay for a cloud service, or worse, recover something which was hosted in a discontinued tool, I'm SOL.
3) Discovery is big too. I can use normal search tools to find things. If something is locked away in a .ai file, a .docx file, or a cloud service, and I lose it, it's likely lost forever.
95% of the cost of most projects is maintenance, and even if I invest 10x the time up-front into making a diagram in a tool like this (e.g. an hour to learn it, tweak it, and get the diagram I want, instead of 5 minutes in my favourite GUI), that will pay much than an hour in dividends down the line. I use Markdown, LaTeX, and similar for large or important documents because, in the long term, they save time.