Fixes #1377 - Document importance of running grunt or npm build

[heatherbooker]

Hopefully this fixes #1377 !

jeanhl had css problems running functional tests because of missing out on running grunt, and I had css problems viewing the site locally - this patch should clarify the importance of having grunt and running grunt or npm run build before moving on to bigger and better things! :)

[heatherbooker]

r? @miketaylr

(I submitted first and it failed the Travis build but...the only thing I changed was the contributing.md file, so I added [ci skip] to the commit msg...I'm not sure if that was the right thing to do >< .)

[zoepage]

@heatherbooker There is an almost finished PR that is fixing a few issues.
Might wanna take a look here? #1351

(Thank you for all of your contributions && your time. You rock!!!)

[miketaylr]

Awesome, thanks @heatherbooker! I just unintentionally created a merge conflict for you, but I can help fix that if you'd like.

[heatherbooker]

Ah ok thanks for the heads up @zoepage !

@miketaylr Ok so you would still like to include these changes? I think I resolved the merge conflict. I'm glad grunt has become a local dependency :)

[miketaylr]

@heatherbooker yeah, I still think it's a good contribution. But let me pass off review to @zoepage, since she's been knee-deep in that recently.

[zoepage]

Great additional info. Thank you :)

[heatherbooker]

:)