documentation: Add "@since 0.10.0" to "withState" haddock docs?

[phlummox]

For users of the library – potentially having to integrate it with other packages, of various versions – it would be handy to be able to see in the Haddock docs when the API has changed. (And thus, if they want to use a newly-introduced function, what version of hspec-wai they'll need to depend on.)

I see withState seems to have been introduced in version 0.10.0. In Haddock docs, this is normally indicated using the @since tag – e.g. @since 0.10.0.

I'd suggest adding @since 0.10.0 to the Haddock documentation for withState. Happy to create a pull request if desired.

[sol]

PRs welcome.