Markdown is a common format for writing readme files on hosting sites like github and bitbucket. Browsers don't have a notice in-built rendering yet though, so I was looking for something to preview the files rather than having to push-to-test on github or bitbucket.
This answer on Unix & Linux Stack Exchange suggested ReText, and I found it was pretty close to what I needed. Ctrl-L shows a live side-by-side preview of the markdown being edited.
On Debian-based distros it can be installed with:
sudo apt-get install retext