add link to @upsetjs/chartjs-chart-venn

[sgratzl]

Awesome Contribution Checklist:

• [x] I have read, and re-read the Contributing Guidelines
• [x] I have searched to ensure the suggested item doesn't exist on this list
• [x] This PR contains only one item

Please Describe Your Addition

allow to rendering venn and euler diagrams

https://github.com/upsetjs/chartjs-chart-venn