Add overview diagram to readme

[nickynicolson]

Github supports mermaid format diagrams embedded in the readme: https://github.blog/2022-02-14-include-diagrams-markdown-files-mermaid/

The other analytical repositories in the OA-WCVP organisation have an overview which demonstrates the data acquisition and processing steps which are used to build the outputs - see eg:

• https://github.com/OA-WCVP/ipni-oa
• https://github.com/OA-WCVP/wcvp-gbif-processing

A similar diagram in this repository would be helpful and will help contribute to the description of methods in the paper