Add graphiql configuration flag to documentation

[schettn]

Hey @MarcDanjou and @schettn, can either of you tell me how to set the new configuration?

I do not mind updating the documentation to reflect the new option, but I do not know what with.

Originally posted by @ckhatton-pvfscaffolding in #72

[ckhatton-pvfscaffolding]

I will try and create a PR to update the documentation. Thank you!