Question: Supergraph Specification License

[andrey-nakin]

Hello

Is there a complete specification of Supergraph architecture that includes definition of federation-specific GraphQL directives, types and queries? And what is its license and terms of usage?

For example, is it free to implement my own gateway that supports the API federation in your way?

This paragraph states that @apollo/subgraph library is under MIT license. What is this library and where can I find its sources and license?