Add OSS sponsorship link to README

Author: SondraE

README.md

@@ -186,3 +186,10 @@ This helps distinguish between the two distinct layers:
   and a smaller set of common-use functions
   that call on the core math as necessary.
   This is the primary API for most users.
+
+_____
+### Sponsor OddBird's OSS Work
+
+At OddBird, we love contributing to the languages & tools developers rely on. We're currently working on polyfills for new Popover & Anchor Positioning functionality, as well as CSS specifications for functions, mixins, and responsive typography. Help us keep this work sustainable and centered on your needs as a developer! We display sponsor logos and avatars on our website.
+
+[Sponsor OddBird's OSS Work](https://opencollective.com/oddbird-open-source)