added recipe for external config file

Author: stryju

docs/README.md

@@ -13,6 +13,7 @@ See [the FAQ](FAQ.md) for the answers to commonly asked questions.
 * [Working with multiple sources in one task](recipes/using-multiple-sources-in-one-task.md)
 * [Mocha test runner with gulp](recipes/mocha-test-runner-with-gulp.md)
 * [Rebuild only files that change](recipes/rebuild-only-files-that-change.md)
+* [Using external config file](recipes/using-external-config-file.md)
 * [Introduction to node.js streams](https://github.com/substack/stream-handbook)
 
 ## Presentations and slides

docs/recipes/using-external-config-file.md

@@ -0,0 +1,49 @@
+# Using external config file
+## bonus: keeping those tasks DRY
+## bonus2: config.json can be used by another task runner, like `Grunt`
+
+---
+
+`config.json`
+
+```json
+{
+  "desktop" : {
+    "src" : [
+      "dev/desktop/js/**/*.js",
+      "!dev/desktop/js/vendor/**"
+    ],
+    "dest" : "build/desktop/js"
+  },
+  "mobile" : {
+    "src" : [
+      "dev/mobile/js/**/*.js",
+      "!dev/mobile/js/vendor/**"
+    ],
+    "dest" : "build/mobile/js"
+  }
+}
+```
+
+---
+
+`gulpfile.js`
+
+```js
+// npm install gulp gulp-uglify
+var gulp   = require('gulp');
+var uglify = require('gulp-uglify');
+var config = require('./config.json');
+
+function doStuff(cfg) {
+  return gulp.src(cfg.src)
+    .pipe(uglify())
+    .pipe(gulp.dest(cfg.dest));
+}
+
+gulp.task('dry', function () {
+  doStuff(config.desktop);
+  doStuff(config.mobile);
+});
+```
+