Document adding markdown pages

[annakrystalli]

Need to unpick how markdown pages are added and served and add to README.

• About page is an example of a markdown page: https://github.com/reprohack/reprohack_site/tree/dev/markdown_pages

When complete, we can add markdown content pages:

• Organisers guidelines Write Organisers Guidelines page #6
• Participant guidelines Write Participants instructions page #7