Add a cookbook on api versionning

[gorghoa]

Regarding the popularity of questions about api versioning (api-platform/core#972, api-platform/core#45, api-platform/api-platform#290, https://symfony-devs.slack.com/archives/C39FKU9AL/p1522244783000485)

And @dunglas’ comment on api-platform/core#45 (comment)

A possible solution is to register a new Resource for the new version referencing the same entity class but with other serialization an validation groups then and play with the OperationFactory to have route similar to v1 (currently the name of a resource must be unique).

Not sure if this is a good idea to introduce a feature like versioning in the bundle. It will very difficult to have a generic enough solution. Maybe a cookbook entry with some insights on how to handle such cases will be better.

I think the subject of api versioning should be present in the doc, even if this is just a “not implemented” page.

Whether the cookbook should give some hint of how api versioning could be achieved with some hacks is debatable. IMO, the doc should provide a fully viable solution or just tell “not implemented, the reccommandation is to create a new project”.

best regards,
Your faithful