Added example in read.md and fixed non-rending var_dump()

It took me a couple of extra minute to understand how to assign a value to DRUPALNAV_MENU_NAME so I added an example in the read.md, and while I was in there caught a spelling mistake. Additionally when I was attempting to var_dump($constant) it didn't render on my screen. I had to add php tags for it to work, so I've added that change here.