Add readme notes about JSON

[sungam3r]

This library, which includes websocket support, specifically is for the GraphQL over HTTP and GraphQL over WebSockets protocol, which requires JSON serialization and deserialization.

Good words to add in README.

Originally posted by @sungam3r in #774 (comment)

[Shane32]

@sungam3r If you feel this comment is appropriate in the readme somewhere, please add a code review suggestion to the readme PR.