More explicit instructions

Running `open http://localhost:3000` would certainly work if the user opened up a new terminal window but won't do anything if it's run immediately after starting the Rails server, which might confuse people new to the command line. I broke it out of code brackets and added an explanatory sentence.
Simplifying to Aidan's suggestion
Update README.md
Merge pull request #12 from ablwr/patch-1

More explicit instructions