improve documentation for generic parameters in README

[scarf005]

Rationale

https://github.com/sindresorhus/ky/blob/b86ff0a4e8d66c363df17612c968c023560f9241/index.test-d.ts#L132 json function can accept type parameters. however issue #312 was raised because README.md did not address which function accepts generic parameters.

Solution

I suggest adding more documentation for functions that accept generic parameters.