How can I test what my readme.md file will look like before committing to github?
Many ways: If you're on a Mac, use Mou.
If you want to test in a browser, you could try StackEdit, as pointed out by @Aaron or Dillinger since Notepag seems to be down now. Personally I use Dillinger since it just works and saves all my documents in my browser's local database.
Atom works nicely out of the box - just open the Markdown file and hit Ctrl+Shift+M to toggle the Markdown preview panel next to it. It handles HTML and images also.
