add example Writing your own Route Enhancers

[andrey1s]

No description provided.

[dbu]

this code would be invalid, we should add a method that does something trivial to make the example complete.

[dbu]

great, thanks! lets complete this with implementing a method in the enhancer that does something simple so its self-contained.

[wouterj]

Let's show that one has to return the new defaults array like this:

public function enhance(array $defaults, Request $request)
{
    // ... tweak the $defaults array

    return $defaults;
}

[dbu]

looks good to me. @wouterj see something that needs to be fixed?
@andrey1s may i ask you to squash the commits into one?

[wouterj]

Thank you @andrey1s! I've merged your pull request into all versions of the documentation and did some minor formatting changes in b586f62