Update maintainer credits in readme and NuGet package authors

[jnm2]

What is the current list of maintainers? @JakeGinnivan, @josephwoodward, @jnm2, @slang25?

@SimonCropp You just published a package in August, are you in or out? :D

I'd like the readme and NuGet authors to reflect this.

[jnm2]

@JakeGinnivan is not a Shouldly org member and is not in @shouldly/core. Should he be removed as an active maintainer, for example from being able to push NuGet packages? We can still keep credits for past significant maintainers.

[JakeGinnivan]

Happy to be removed, if I ever find the need to jump back in I can just ask. Not doing a lot of .net (though am working at a company now who does after 5 years of nodejs only).

[SimonCropp]

@jnm2 yeah. i am happy to be in for now.

[slang25]

My availability tends to be patchy, but I'd like to stay active. I don't contribute to a level that would warrant mention in the README however 😄