Add the method: AddCustomProperty

[paatrick]

Hi,

There is a very useful method that should be in the documentation.

I spent hours trying to solve a problem, I needed to add a metadata and I couldn't find it anywhere.

I could add the method: AddCustomProperty in the doc, so that other people don't experience the same problem as me.

Thanks for the package.

[finwe]

Hi, thanks for a proposition, can you prepare a Pull Request?