Show required serde dependency in README

[hummingly]

Before using this crate I've never had used GraphQL before, so I got many errors and didn't realize that serde was also needed to use the derive plus I didn't need serde itself because I was using reqwest's json feature. Showing a Cargo.toml snippet on the README would make it easier to get started.

[arjunyel]

What does your Cargo.toml look like?

[hummingly]

This was a really long time ago but I think I had to add serde with derive feature enabled.