[WIP/RFC] example for using standalone service providers

to make the service providers available to non silex projects.

This is a RFC neither finished nor complete, not to be merged.

| Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [yes]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | []
| License       | MIT
| Doc PR        | []