Updating docs

[oojacoboo]

It's about time for this lib to support it's own docs. Please see https://github.com/thecodingmachine/graphqlite/issues/376

GraphQLite core isn't aware of the particulars of the Symfony implementation. In the core docs, a link to the Symfony bundle docs will be promoted.

Please provide an update on this with the preferred link as soon as possible, so core docs can be updated accordingly.