Updated Readme: debugging Gulp tasks

Adds documentation on how to debug Gulp tasks.

Solves #447 and #348.

**Unix:**
```sh
node-debug $(which gulp) task
```

**Window:**
```sh
node-debug "C:\Users\user\AppData\Roaming\npm\node_modules\gulp\bin\gulp.js" task
```
We use the convention of prepending `$ ` in front of commands run via command line, see examples in other places of the README.