Add examples of the external_link() function in run_example and README

[cjrace]

Describe the solution you'd like

Some examples of it being used both with and without the warning for users to see, highlighting the guidance on it's usage in the documentation.

Describe alternatives you've considered

None.

Additional context

Mostly a reminder to myself to pick up when I'm back, following on from #115 - didn't do in that PR to avoid conflicts with my other PRs editing the example app.