Add default_country country to usage documentation

[raspberryswwwirl]

I've been using this module for a long time, but mostly for URL type and such; it's only recently that I've had a use for the Phone type in my work. One important configuration value that should be captured in the documentation is the default_country static property here:

gorriecoe\Link\View\Phone

and referenced here.

For clarity, this should be added as part of the default _config/config.yml

gorriecoe\Link\View\Phone:
  default_country: NZ