Link from CONTRIBUTING to the code of conduct.

Contributes to https://github.com/dart-lang/site-www/issues/1792.

Closes https://github.com/dart-lang/sdk/pull/40024
https://github.com/dart-lang/sdk/pull/40024

GitOrigin-RevId: a05469ef645eaaedac29a9aba141b083dcfbd378
Change-Id: Idef17931fe63038ba09873f1257ad0077e51863a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/130731
Reviewed-by: Michael Thomsen <mit@google.com>
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 80cdf8e..5669836 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -115,3 +115,5 @@
 ## The small print
 
 Contributions made by corporations are covered by a different agreement than the one above, the [Software Grant and Corporate Contributor License Agreement](http://code.google.com/legal/corporate-cla-v1.0.html).
+
+We pledge to maintain an open and welcoming environment. For details, see our [code of conduct](https://dart.dev/code-of-conduct).