Add section on financial support in contributing.md

contributing.md

@@ -18,6 +18,7 @@ free customer service.
 *   [Ecosystem](#ecosystem)
 *   [Questions](#questions)
 *   [Contributions](#contributions)
+    *   [Financial support](#financial-support)
     *   [Improve documentation](#improve-documentation)
     *   [Improve issues](#improve-issues)
     *   [Give feedback on issues](#give-feedback-on-issues)
@@ -60,6 +61,16 @@ Jump in there and lurk, talk to us, and help others.
 
 There’s several ways to contribute, not just by writing code.
 
+### Financial support
+
+It’s possible to support us financially by becoming a backer or sponsor through
+[Open Collective][collective].
+With this support, we can pay for project leadership, finance non-coding work,
+or to do fun things for the community like getting stickers for contributors.
+You’ll be helping unified’s maintainers manage and improve existing projects,
+and additionally support our work to develop new and exciting projects, such
+as [micromark][].
+
 ### Improve documentation
 
 As a user of this project you’re perfect for helping us improve our docs.
@@ -140,4 +151,8 @@ prevent you from doing unnecessary work.
 
 [unit-test]: https://twitter.com/sindresorhus/status/579306280495357953
 
+[collective]: https://opencollective.com/unified
+
+[micromark]: https://github.com/micromark/micromark
+
 [chat]: https://spectrum.chat/unified