Clarify the use of HTTP GET and POST request

from [GraphQL over HTTP Spec](https://graphql.github.io/graphql-over-http/draft/#sec-Request)
> A server MUST accept POST requests, and MAY accept other HTTP methods, such as GET.

For semantics and performance (caching, etc.), it seems to me that it is recommended to use `GET` for the query operation.

If the options are not equivalent, it is necessary to clarify which is more appropriate.