Document how to write a Truth extension under Kotlin

It's pretty simple (and pretty similar to Java), but it's always nice for users to have an example.

\[edit: I now have [a draft example](https://github.com/google/truth/issues/661#issuecomment-1452472936) below.\]

We could also consider suggesting that every `Subject.Factory` come with an extension-method "overload" of `StandardSubjectBuilder.that` (#660).

\[edit: See also https://github.com/google/truth/issues/660#issuecomment-803529493, which suggests that we may be able to make extensions easier to write under Kotlin than under Java.\]