Added info and example to the grunt.config.merge doc blurb.

### Tiny add to the ```grunt.config.merge``` documentation.
I was trying to append a target to a task and it took me a while to figure out this can be done with ```grunt.config.merge```. I wasn't so sure about how to do it either so a simple example would have helped too. I propose this tiny add to fix that.

And just for reference, here is my code:

```coffeescript
  grunt.registerTask "samples", ->
    samples = grunt.file.readJSON("package.json").samples
    return unless samples.enabled
    grunt.config "jade",
      samples: files: [
        expand: true
        cwd: samples.path
        src: ["*.jade"]
        dest: samples.local
        ext: ".php"
      ]
    grunt.config.merge
      watch:
        files: [samples.path + "*.jade"]
        tasks: ["samples"]
    grunt.task.run "jade"
```