Document incremental builds

[yocontra]

vinyl-fs has support for a new since parameter to only spit out modified files. we should document how to use this (possibly with a registry?) with the new task system

[yocontra]

also new and relevant gulpjs/vinyl#39

[dinoboff]

#937 document the gulp.src since option.

I can document the new api to get the last run time of a task (gulpjs/undertaker#9) once it lands in Gulp.

[phated]

Thanks to the efforts by @dinoboff this is implemented and documented in the 4.0 branch. Closing.