Improve example instructions

I tried to run the commands in the example's README without first running `npm install` inside the root directory and got an error. It's a n00b mistake, but a frustrating one. Let's make that initial experience smoother.

For reference, here's the error I got:
```
$ broccoli build dist

module.js:340
    throw err;
          ^
Error: Cannot find module 'broccoli-es6-module-filter'
    at Function.Module._resolveFilename (module.js:338:15)
    at Function.Module._load (module.js:280:25)
    at Module.require (module.js:364:17)
    at require (module.js:380:17)
    at Object.<anonymous> (/Users/strathmeyer/Development/broccoli-dist-es6-module/index.js:1:86)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Module.require (module.js:364:17)
```