gi-gio-2.0.16: ChangeLog.md

### 2.0.16

+ The return value of [`volumeGetMount`](https://hackage.haskell.org/package/gi-gio/docs/GI-Gio-Interfaces-Volume.html#v:volumeGetMount) is nullable, but was not marked as such.

### 2.0.15

+ Remove enable-overloading flags, and use instead explicit CPP checks for 'haskell-gi-overloading-1.0', see [how to disable overloading](https://github.com/haskell-gi/haskell-gi/wiki/Overloading\#disabling-overloading).