Add "run npm test" in the "Steps" section of README.md.

Before submitting #254, I didn't think about running the whole tests suite. As a consequence, Travis' build failed and I missed the fact that my rule should have been set to "ignore" by default. I see other pull requests for which Travis build's fails (like #251), and I think adding a `npm test` step in the README.md file could help avoid these issues.