Broken links in guidelines

[MattMS]

The gulp-header and gulp-footer links on the guidelines page result in a 404.

Should these be updated to the NPM package pages for header and footer instead?

I got those NPM links from a quick Google search, and have not used them before, so I am not sure if they are correct.

[phated]

Please submit a PR. Thanks!

[benhalverson]

@phated I can make a quick PR for this. Should the gulp-concat link also point to the npm link?