Docs: Add a "BrowserSync with Gulp 4" recipe (#1659)

docs/recipes/minimal-browsersync-setup-with-gulp4.md

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+# Minimal BrowserSync setup with Gulp 4
+
+[BrowserSync](https://www.browsersync.io/) is a great tool to streamline
+the development process with the ability to reflect code changes instantaneously
+in the browser through live-reloading. Setting up a live-reloading
+BrowserSync server with Gulp 4 is very clean and easy.
+
+## Step 1: Install the dependencies
+
+```
+npm install --save-dev browser-sync
+```
+
+## Step 2: Setup the project structure
+
+```
+src/
+  scripts/
+    |__ index.js
+dist/
+  scripts/
+index.html
+gulpfile.babel.js
+```
+
+The goal here is to be able to:
+- Build the source script file in `src/scripts/`, e.g. compiling with babel, minifying, etc.
+- Put the compiled version in `dist/scripts` for use in `index.html`
+- Watch for changes in the source file and rebuild the `dist` package
+- With each rebuild of the `dist` package, reload the browser to immediately reflect the changes
+
+## Step 3: Write the gulpfile
+
+The gulpfile could be broken in 3 parts.
+
+### 1. Write the task to prepare the dist package as usual
+
+Refer to the main [README](https://github.com/gulpjs/gulp/blob/4.0/README.md#use-last-javascript-version-in-your-gulpfile)
+for more information.
+
+```javascript
+import babel from 'gulp-babel';
+import concat from 'gulp-concat';
+import del from 'del';
+import gulp from 'gulp';
+import uglify from 'gulp-uglify';
+
+const paths = {
+  scripts: {
+    src: 'src/scripts/*.js',
+    dest: 'dist/scripts/'
+  }
+};
+
+const clean = () => del(['dist']);
+
+function scripts() {
+  return gulp.src(paths.scripts.src, { sourcemaps: true })
+    .pipe(babel())
+    .pipe(uglify())
+    .pipe(concat('index.min.js'))
+    .pipe(gulp.dest(paths.scripts.dest));
+}
+```
+
+### 2. Setup the BrowserSync server
+
+And write the tasks to serve and reload the server accordingly.
+
+```javascript
+import browserSync from 'browser-sync';
+const server = browserSync.create();
+
+function reload(done) {
+  server.reload();
+  done();
+}
+
+function serve(done) {
+  server.init({
+    server: {
+      baseDir: './'
+    }
+  });
+  done();
+}
+```
+
+### 3. Watch for source change, rebuild the scripts and reload the server
+
+This is trivially accomplished with `gulp.series`
+
+```javascript
+const watch = () => gulp.watch(paths.scripts.src, gulp.series(scripts, reload));
+```
+
+## Step 4: Bring it all together
+
+The last step is to expose the default task
+
+```javascript
+const dev = gulp.series(clean, scripts, serve, watch);
+export default dev;
+```
+
+And profit
+
+```bash
+$ gulp
+```
+
+Now if you go to [http://localhost:3000](http://localhost:3000), which is the default address of the
+BrowserSync server, you will see that the end result in the browser is updated everytime you change
+the content of the source file. Here is the whole gulpfile:
+
+```javascript
+import babel from 'gulp-babel';
+import concat from 'gulp-concat';
+import del from 'del';
+import gulp from 'gulp';
+import uglify from 'gulp-uglify';
+import browserSync from 'browser-sync';
+
+const server = browserSync.create();
+
+const paths = {
+  scripts: {
+    src: 'src/scripts/*.js',
+    dest: 'dist/scripts/'
+  }
+};
+
+const clean = () => del(['dist']);
+
+function scripts() {
+  return gulp.src(paths.scripts.src, { sourcemaps: true })
+    .pipe(babel())
+    .pipe(uglify())
+    .pipe(concat('index.min.js'))
+    .pipe(gulp.dest(paths.scripts.dest));
+}
+
+function reload(done) {
+  server.reload();
+  done();
+}
+
+function serve(done) {
+  server.init({
+    server: {
+      baseDir: './'
+    }
+  });
+  done();
+}
+
+const watch = () => gulp.watch(paths.scripts.src, gulp.series(scripts, reload));
+
+const dev = gulp.series(clean, scripts, serve, watch);
+export default dev;
+```