clarify documentation for select() #77

[simbabque]

This also modernises the test slightly.

Closes #77. It is not a proper fix though. The ticket stated it's not possible to select multiple additional values at once. I don't think there's a clean way of doing this in terms of syntax. It's already quite a complex, confusing method. I opted to explain how to do it the long way in your own code instead.

[oalders]

Test::Deep now depends on 5.12, so we need to deal with that in some way in order for the tests to pass.

[simbabque]

365 fixes the Test::Deep issue. We can merge this once it's landed.