Add docs for units for function args

Hi,

This is an amazing package but I'm trying to figure out how to specify function signatures:

```
foo(t::Temperature, r::Radian) = (2t, 5r)
foo(20°C, 1.5rad)
```
The above is incorrect but I'm wondering what is the best way to define type parameters for things like any numeric value that is a temperature or any numeric value that is in Celcius.

Glen