Docs: Add "Project structure" section to CONTRIBUTING.md (#1859)

Author: callumacrae

CONTRIBUTING.md

@@ -28,6 +28,53 @@ of the content
 - formatting: help keep content easy to read with consistent formatting
 - code: Fix issues or contribute new features to this or any related projects
 
+# Project structure
+
+Gulp itself is tiny: index.js contains [very few lines of code](https://github.com/gulpjs/gulp/blob/4.0/index.js).
+It is powered by a few other libraries which each handle a few specific tasks
+each.
+
+You can view all issues with the "help wanted" label across all gulp projects
+here: https://github.com/issues?utf8=%E2%9C%93&q=is%3Aopen+is%3Aissue+user%3Agulpjs+label%3A%22help+wanted%22+
+
+## Undertaker: task management
+
+Undertaker handles task management in Gulp: the `gulp.task()`, `gulp.series()`
+and `gulp.parallel()` functions. `gulp.series()` and `gulp.parallel()` are in
+turn powered by Bach.
+
+- https://github.com/gulpjs/undertaker
+- https://github.com/gulpjs/bach
+
+## vinyl-fs: file streams
+
+vinyl-fs powers the `gulp.src()` and `gulp.dest()` functions: they take files
+and globs specified by the user, turns them into a stream of file objects,
+and then puts them back into the filesystem when `gulp.dest()` is called.
+
+The file objects themselves are vinyl objects: that's another library (a simple
+one!)
+
+- https://github.com/gulpjs/vinyl-fs
+- https://github.com/gulpjs/vinyl
+
+## chokidar: file watching
+
+`gulp.watch()` is using chokidar for file watching. It's actually wrapped in a
+small library on the gulp organization, glob-watcher.
+
+- https://github.com/paulmillr/chokidar
+- https://github.com/gulpjs/glob-watcher
+
+## gulp-cli: running gulp
+
+Finally, we have gulp-cli. This uses liftoff to take what people run in the
+command line and run the correct tasks. It works with both gulp 4 and older
+versions of gulp.
+
+- https://github.com/gulpjs/gulp-cli
+- https://github.com/js-cli/js-liftoff
+
 # Conduct
 
 We are committed to providing a friendly, safe and welcoming environment for