Move rfcs/ to graphql/graphql-wg (#884)

See graphql/graphql-wg#741 which is the receiving side of this move.

This rationale is to limit the change history of the spec repo to only spec changes and co-locate the rfc working documents with the rest of the working group's files.

See this issue for more context: graphql/graphql-wg#489

Author: leebyron