Migrate `pump` documentation

We should migrate the `pump` documentation from `gulp-uglify` to here, since it's relevant for more than just `gulp-uglify` and is probably a better best practice for streams in general.

https://github.com/terinjokes/gulp-uglify/tree/master/docs/why-use-pump

@phated can you tag and assign this to me?
