Create ErrorDialog.pod

In the list of Tk modules, there is Tk::ErrorDialog. Also, when users will handle errors using Tk::ErrorDialog, they will put a use Tk::ErrorDialog statement in their code. But no documentation for Tk/ErrorDialog.pm can be found. Instead, someone has to search and find Tk::Error.
Yes, it's not that far away :)
But here is a proposal how some documentation could look like to point at least to Tk::Error.
Merge pull request #6 from pktm/patch-1

Create ErrorDialog.pod