Adding docs for running task steps per folder

docs/recipes/running-task-steps-per-folder

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+# Generating a file per folder
+
+If you have a set of folders, and wish to perform a set of tasks on each, for instance
+
+   /scripts
+   /scripts/jquery/
+   /scripts/angularjs/
+
+and want to end up with
+
+   /scripts
+   /scripts/jquery.min.js
+   /scripts/angularjs.min.js
+
+and so on, you need to know a little more NodeJS and event streams. 
+
+``` javascript
+var 
+  fs = require('fs')
+  ,es = require('event-stream')
+  ,gulp = require('gulp')
+  ,uglify = require('gulp-uglify');
+
+var scriptsPath = './src/scripts/';
+
+function getFolders(dir){
+    return fs.readdirSync(dir)
+      .filter(function(file){
+        return fs.statSync(dir + '/' + file).isDirectory();
+      });
+}
+
+gulp.task('scripts', function() { 
+   var folders = getFolders(scriptsPath);
+
+   return es.concat.apply(null,(folders.map(function(folder) {
+      return gulp.src(scriptsPath + folder + '/*.js')
+        .pipe(uglify())
+        .pipe(rename( folder + '.min.js'))
+        .pipe(gulp.dest(scriptsPath));
+   })));
+});
+```
+
+A few notes with this:
+
+- folders.map - executes the function once per folder, and returns the async stream
+- es.concat - combines the streams and ends only when all streams emitted end
+- the call to .apply(null, args) is needed as es.concat expects arguments not an array