Updating the documentation for eg. GetFirstOrDefault

[chudi]

In extension is several methods, that aren't described in Readme, but are useful eg. GetFirstOrDefault

[bjbr-dev]

Thanks for the feedback! I'm currently overhauling the entire API (and renaming the project as you can probably see) to make it a lot easier to start using, and I'll double down on the documentation as soon as I can!