Add documentation for adding TypeScript type to a custom type

[thesunny]

SuperStruct is able to return the proper typing information for a custom type but the documentation does not indicate how to do this.

Currently the documentation shows this:

const Email = struct('Email', isEmail)

Augment the documentation to show generics, possibly as a second example for TypeScript users only so as not to confuse JavaScript users:

const Email = struct<string>('Email', isEmail)

[thesunny]

Hi @ianstormtaylor,

I will PR most of these when I have time.

[ianstormtaylor]

@thesunny nice, that would be great!

I think maybe there should be an additional section in the guide called something like "Using TypeScript" and it would have these kinds of things. The other one is the StructType inferring utility.

[thesunny]

PR #393