Add documentation about adding a new product to an existing provider

[amarandon]

No description provided.

[amarandon]

@alambare I've pushed an initial version of the requested document where I show how to add an entry to product_types.yaml. Is there anything else do do? Until recently I would have thought that it would also be necessary to list the product in providers.yml but while working on another pull request I was told that this actually isn't required. If that is indeed not required, then I'm wondering how EODag can decide what providers support what product types.