Add TypeFactory feature to the readme #62

One of the features that makes assertive-ts different is the TypeFactory feature. We should highlight this on the README.md and add some examples in the Usage section. Code examples will also make visible the exported TypeFactories object which provides some basic factories.

Additionally, we should explain what to do when the expected value is of type any. Because of the function overload, we hit a TypeScript limitation, and the first overload matches (because any does extend from the first overload's type). The best approach for these cases is to use the .asType(TypeFactory) method.