Ask HN: How to document things I learned while working on a side project?

I love to work on side projects as they allow me learn a lot of things. But most of the stuff I thus manage to learn ... is forgotten. And that is very disappointing. In my opinion, this happens because -- while trying to solve an issue -- I don't go directly from the Source Problem to the Final Solution. Instead, my path goes in loops; from what I initially thought was the problem to what I initially thought would be the solution -- getting closer to the Source Problem and the Final Solution with each Stack Overflow answer, blog post or piece of documentation I read. When I finally find the Optimal Path, my mind is too fuzzy to remember exactly what was the real problem, let alone the solution. To get rid of all of this, I decided to create a JOURNAL.md file, where I document everything about that Optimal Path, so that I can wrap my mind around it. I keep this file in my project's root and commit it with the rest of the project. Thus, I can keep each commit with the stuff I learned while building it -- while keeping the document easily accessible. My question is: Is there a better way to document this kind of stuff? Should I be adding it to the commit message? Any recommendation? What's your solution?