Document how to make Options required

I read through the docs and couldn't find how to make options required.
After playing around with the API I found the `IsRequired` property, Can this be documented in the Wiki?

Also would it be worth adding `isRequired` to an Option constructor for easier discovery? It seems to me it's might be a very common use-case to have certain flags required.

At the moment the best way of specifying required I found looks like this:
```
new Option("--aws-key", "AWS API Key") { IsRequired = true },
```