Docs: Add "Async Completion" documentation

docs/getting-started/4-async-completion.md

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+<!-- front-matter
+id: async-completion
+title: Async Completion
+hide_title: true
+sidebar_label: Async Completion
+-->
+
+# Async Completion
+
+Node libraries handle asynchronicity in a variety of ways. The most common pattern is [error-first callbacks][node-api-error-first-callbacks], but you might also encounter [streams][stream-docs], [promises][promise-docs], [event emitters][event-emitter-docs], [child processes][child-process-docs], or [observables][observable-docs]. Gulp tasks normalize all these types of asynchronicity.
+
+## Signal task completion
+
+When a stream, promise, event emitter, child process, or observable is returned from a task, the success or error informs gulp whether to continue or end. If a task errors, gulp will end immediately and show that error.
+
+When composing tasks with `series()`, an error will end the composition and no further tasks will be executed. When composing tasks with `parallel()`, an error will end the composition but the other parallel tasks may or may not complete.
+
+### Returning a stream
+
+```js
+const { src, dest } = require('gulp');
+
+function streamTask() {
+  return src('*.js')
+    .pipe(dest('output'));
+}
+
+exports.default = streamTask;
+```
+
+### Returning a promise
+
+```js
+function promiseTask() {
+  return Promise.resolve('some ignored value');
+}
+
+exports.default = promiseTask;
+```
+
+### Returning an event emitter
+
+```js
+const { EventEmitter } = require('events');
+
+function eventEmitterTask() {
+  const emitter = new EventEmitter();
+  // Emit has to happen async otherwise gulp isn't listening yet
+  setTimeout(() => emitter.emit('finish'), 250);
+  return emitter;
+}
+
+exports.default = eventEmitterTask;
+```
+
+### Returning a child process
+
+```js
+const { exec } = require('child_process');
+
+function childProcessTask() {
+  return exec('date');
+}
+
+exports.default = childProcessTask;
+```
+
+### Returning an observable
+
+```js
+const { Observable } = require('rxjs');
+
+function observableTask() {
+  return Observable.of(1, 2, 3);
+}
+
+exports.default = observableTask;
+```
+
+### Using an error-first callback
+
+If nothing is returned from your task, you must use the error-first callback to signal completion. The callback will be passed to your task as the only argument - named `done()` in the examples below.
+
+```js
+function callbackTask(done) {
+  // `done()` should be called by some async work
+  done();
+}
+
+exports.default = callbackTask;
+```
+
+To indicate to gulp that an error occurred in a task using an error-first callback, call it with an `Error` as the only argument.
+
+```js
+function callbackError(done) {
+  // `done()` should be called by some async work
+  done(new Error('kaboom'));
+}
+
+exports.default = callbackError;
+```
+
+However, you'll often pass this callback to another API instead of calling it yourself.
+
+```js
+const fs = require('fs');
+
+function passingCallback(done) {
+  fs.access('gulpfile.js', done);
+}
+
+exports.default = passingCallback;
+```
+
+## No synchronous tasks
+
+Synchronous tasks are no longer supported. They often led to subtle mistakes that were hard to debug, like forgetting to return your streams from a task.
+
+When you see the _"Did you forget to signal async completion?"_ warning, none of the techniques mentioned above were used. You'll need to use the error-first callback or return a stream, promise, event emitter, child process, or observable to resolve the issue.
+
+## Using async/await
+
+When not using any of the previous options, you can define your task as an [`async` function][async-await-docs], which wraps your task in a promise. This allows you to work with promises synchronously using `await` and use other synchronous code.
+
+```js
+const fs = require('fs');
+
+async function asyncAwaitTask() {
+  const { version } = fs.readFileSync('package.json');
+  console.log(version);
+  await Promise.resolve('some result');
+}
+
+exports.default = asyncAwaitTask;
+```
+
+[node-api-error-first-callbacks]: https://nodejs.org/api/errors.html#errors_error_first_callbacks
+[stream-docs]: https://nodejs.org/api/stream.html#stream_stream
+[promise-docs]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_promises
+[event-emitter-docs]: https://nodejs.org/api/events.html#events_events
+[child-process-docs]: https://nodejs.org/api/child_process.html#child_process_child_process
+[observable-docs]: https://github.com/tc39/proposal-observable/blob/master/README.md
+[async-await-docs]: https://developers.google.com/web/fundamentals/primers/async-functions