Proposal: Do not require query as a root operation type

[The official spec](http://facebook.github.io/graphql/June2018/#sec-Root-Operation-Types) states:

> A schema defines the initial root operation type for each kind of operation it supports: query, mutation, and subscription; this determines the place in the type system where those operations begin.
The query root operation type must be provided and must be an Object type.
The mutation root operation type is optional; if it is not provided, the service does not support mutations. If it is provided, it must be an Object type.
Similarly, the subscription root operation type is also optional; if it is not provided, the service does not support subscriptions. If it is provided, it must be an Object type.

Why must the `query` root operation be provided? It seems to me that it should be perfectly legal to have an API that only contains a mutation or subscription. Especially with the advent of subscriptions, which are similar to queries in that they're only meant for retrieving data rather than modifying it, I'm not sure I see a purpose in requiring a query specifically.

Some more details on my specific use case, to provide some more context: I'm employing [schema stitching](https://www.apollographql.com/docs/graphql-tools/schema-stitching.html) to merge numerous disjoint schemas together and present them as one. I have a subscription that I want to host in its own API, since doesn't logically belong in any of my other APIs; but the GraphQL implementation I'm using expects that a query will exist in the schema, forcing me to add an unnecessary query into my schema. Removing this stipulation from the spec (as well as from the implementation) would allow for this use case.

This stipulation is mentioned in #218, but in a slightly different context, and I think it warrants further discussion.