Added form cookbook article about transforming delimited strings to arra...

As suggested in https://github.com/symfony/symfony/pull/10015

The above PR was rejected but it is still a common requirement so I created a package and @weaverryan suggested creating a cookbook article.

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | >= 2.2
| Fixed tickets | n/a
we decided to use `composer require dantleech/symfony-form-array-to-delimited-string-transformer:0.1.0` instead of "adding to composer.json".