docs(pump): move pump documentation from gulp-uglify

Closes #1791.

docs/README.md

@@ -4,6 +4,7 @@
 * [API documentation](API.md) - Learn the ins and outs of using gulp
 * [CLI documentation](CLI.md) - Learn how to call tasks and use compilers
 * [Writing a Plugin](writing-a-plugin/README.md) - So you're writing a gulp plugin? Go here for the essential dos and don'ts.
+* [Why Use Pump?](why-use-pump/README.md) - Why you should use the `pump` module instead of calling `.pipe` yourself.
 * [Spanish documentation][SpanishDocs] - gulp en Español.
 * [Simplified Chinese documentation][SimplifiedChineseDocs] - gulp 简体中文文档.
 * [Korean documentation][KoreanDocs] - gulp 한국어 참조 문서.

docs/why-use-pump/README.md

@@ -0,0 +1,122 @@
+# Why Use Pump?
+
+When using `pipe` from the Node.js streams, errors are not propagated forward
+through the piped streams, and source streams aren’t closed if a destination
+stream closed. The [`pump`][pump] module normalizes these problems and passes
+you the errors in a callback.
+
+## A common gulpfile example
+
+A common pattern in gulp files is to simply return a Node.js stream, and expect
+the gulp tool to handle errors.
+
+```javascript
+// example of a common gulpfile
+var gulp = require('gulp');
+var uglify = require('gulp-uglify');
+
+gulp.task('compress', function () {
+  // returns a Node.js stream, but no handling of error messages
+  return gulp.src('lib/*.js')
+    .pipe(uglify())
+    .pipe(gulp.dest('dist'));
+});
+```
+
+![pipe error](pipe-error.png)
+
+There’s an error in one of the JavaScript files, but that error message is the
+opposite of helpful. You want to know what file and line contains the error. So
+what is this mess?
+
+When there’s an error in a stream, the Node.js stream fire the 'error' event,
+but if there’s no handler for this event, it instead goes to the defined
+[uncaught exception][uncaughtException] handler. The default behavior of the
+uncaught exception handler is documented:
+
+> By default, Node.js handles such exceptions by printing the stack trace to
+> stderr and exiting.
+
+## Handling the Errors
+
+Since allowing the errors to make it to the uncaught exception handler isn’t
+useful, we should handle the exceptions properly. Let’s give that a quick shot.
+
+```javascript
+var gulp = require('gulp');
+var uglify = require('gulp-uglify');
+
+gulp.task('compress', function () {
+  return gulp.src('lib/*.js')
+    .pipe(uglify())
+    .pipe(gulp.dest('dist'))
+    .on('error', function(err) {
+      console.error('Error in compress task', err.toString());
+    });
+});
+```
+
+Unfortunately, Node.js stream’s `pipe` function doesn’t forward errors through
+the chain, so this error handler only handles the errors given by
+`gulp.dest`. Instead we need to handle errors for each stream.
+
+```javascript
+var gulp = require('gulp');
+var uglify = require('gulp-uglify');
+
+gulp.task('compress', function () {
+  function createErrorHandler(name) {
+    return function (err) {
+      console.error('Error from ' + name + ' in compress task', err.toString());
+    };
+  }
+
+  return gulp.src('lib/*.js')
+    .on('error', createErrorHandler('gulp.src'))
+    .pipe(uglify())
+    .on('error', createErrorHandler('uglify'))
+    .pipe(gulp.dest('dist'))
+    .on('error', createErrorHandler('gulp.dest'));
+});
+```
+
+This is a lot of complexity to add in each of your gulp tasks, and it’s easy to
+forget to do it. In addition, it’s still not perfect, as it doesn’t properly
+signal to gulp’s task system that the task has failed. We can fix this, and we
+can handle the other pesky issues with error propogations with streams, but it’s
+even more work!
+
+## Using pump
+
+The [`pump`][pump] module is a cheat code of sorts. It’s a wrapper around the
+`pipe` functionality that handles these cases for you, so you can stop hacking
+on your gulpfiles, and get back to hacking new features into your app.
+
+```javascript
+var gulp = require('gulp');
+var uglify = require('gulp-uglify');
+var pump = require('pump');
+
+gulp.task('compress', function (cb) {
+  pump([
+      gulp.src('lib/*.js'),
+      uglify(),
+      gulp.dest('dist')
+    ],
+    cb
+  );
+});
+```
+
+The gulp task system provides a gulp task with a callback, which can signal
+successful task completion (being called with no arguments), or a task failure
+(being called with an Error argument). Fortunately, this is the exact same
+format `pump` uses!
+
+![pump error](pump-error.png)
+
+Now it’s very clear what plugin the error was from, what the error actually was,
+and from what file and line number.
+
+[pump]: https://github.com/mafintosh/pump
+[uncaughtException]: https://nodejs.org/api/process.html#process_event_uncaughtexception