Document graphql dependency requirement

[lensbart]

Although graphql will often already be installed in projects using this module, this isn’t necessarily the case (e.g. when sending queries using graphql-request. If graphql is not installed, attempting to use this module will throw the following error:

Error: Cannot find module 'graphql/language'
Require stack:
(...)

which can be fixed by installing graphql.

[lensbart]

Also, the queries need to be wrapped in print, imported as { print } from 'graphql'