Add details about the .gqlconfig file in readme

[kumarharsh]

What format should the file be in? Also, what do the fields path, dir, schema mean?

[enaeseth]

@kumarharsh there's a .gqlconfig in the project tests:

{
  schema: {
    files: "schema/**/*.gql"
  }
}

no idea if anything else can go in there – schema.files is the only member that gets validated.