Fixup usage docs (sources this time)

Updates the docs/usage.md source file so README.md the change won't be overwritten.

I also noticed that `require('gulp-jshint/node_modules/map-stream')` doesn't work with gulp-jshint version 1.8.4 because they apparently replaced the map-stream dependency with through2: https://github.com/spenceralger/gulp-jshint/commit/a817a112a2f0df9eee0889a5ba73f7f437ebaa81

I split 'map-stream' out into a dev dependency. Let me know if you'd prefer that I split that out into a distinct PR.