Update code of conduct (facebook#21251)

We are updating our code of conduct to include activities outside the project.

https://developers.facebook.com/blog/post/2021/04/12/facebook-open-source-ally/

CODE_OF_CONDUCT.md

@@ -52,6 +52,10 @@ project e-mail address, posting via an official social media account, or acting
 as an appointed representative at an online or offline event. Representation of
 a project may be further defined and clarified by project maintainers.
 
+This Code of Conduct also applies outside the project spaces when there is a
+reasonable belief that an individual's behavior may have a negative impact on
+the project or its community.
+
 ## Enforcement
 
 Instances of abusive, harassing, or otherwise unacceptable behavior may be