A review of Shiny Frog's Bear Writer, which I really like and assessed to see if I could integrate it into my GTD process. I think it's ideal for note-taking, short-form writing (blog posts and such) and journaling, although some people might not like the subscription model it uses. I think it's worth looking at if you want a Markdown-based notes app.
The process I used to install Jekyll on MacOS 10.11+ (actually 10.12.6) after encountering errors such as 'You don't have write permissions', 'public_suffix requires Ruby version >= 2.1', 'Your Xcode (7.2.1) is too outdated' and 'rbenv shell command not found'. It was a bit of a nightmare, but I got there in the end.