Update External-Resources.md

[kiramclean]

I clicked the "edit" button from the docs to add a project I worked on with Swirrl, the ONS Structural Search demo. I think it's a decent example of a non-trivial re-frame project and was recently made public, so wanted to offer it here as an addition. Let me know if this is the wrong place or way to submit an update to this list! Thank you for all the amazing work that's gone into this project.

[superstructor]

Very cool @kiramclean Thanks for sharing.