Update priority-pathogens gh-pages home page text

[tristan-myles]

To do this, the assignee would need to update the main README.md. The README has not been updated since the living reviews were moved to priority pathogens. Ideally, this should be updated to indicate this priority-pathogens is the home of the living reviews and analysis that's been done and point the user to epireview for the data.

[tristan-myles]

Here is a is a potential suggested README. It was drafted prior to some updates to the epireview README, but is largely similar. I think it makes sense for both READMEs to be be similar and to have a synergy between them?