Add archiving section to README

Added archiving information with Zenodo DOI badge.

Author: LewisAJones

README.md

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 Current edition: 1 (2023)
 
 This repository is for the Palaeoverse Workshop and Hackathon site. It includes all the required resources and information for the event. The site is setup with Github Actions for automatic deployment upon committing to the repository. Instructors should edit the relevant `.qmd` files using plain [Markdown](https://www.markdownguide.org).
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+## Archiving
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+These materials are archived on Zenodo: [![DOI](https://zenodo.org/badge/615046266.svg)](https://doi.org/10.5281/zenodo.20657194)