docs: Update backer img (#573)

Author: iamkun

README.md

@@ -110,6 +110,10 @@ This project exists thanks to all the people who contribute.
 Please give us a 💖 star 💖 to support us. Thank you.
 
 And thank you to all our backers! 🙏
+
+<a href="https://opencollective.com/dayjs/backer/0/website?requireActive=false" target="_blank"><img src="https://opencollective.com/dayjs/backer/0/avatar.svg?requireActive=false"></a>
+<a href="https://opencollective.com/dayjs/backer/1/website?requireActive=false" target="_blank"><img src="https://opencollective.com/dayjs/backer/1/avatar.svg?requireActive=false"></a>
+<a href="https://opencollective.com/dayjs/backer/2/website?requireActive=false" target="_blank"><img src="https://opencollective.com/dayjs/backer/2/avatar.svg?requireActive=false"></a>
 <a href="https://opencollective.com/dayjs#backers" target="_blank"><img src="https://opencollective.com/dayjs/contributors.svg?width=890" /></a>
 
 ## License