Document `artifacts:public` keyword and feature flag

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/295139

doc/ci/yaml/README.md

@@ -3097,6 +3097,32 @@ job:
       - path/*xyz/*
 ```
 
+#### `artifacts:public`
+
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/49775) in GitLab 13.8
+> - It's [deployed behind a feature flag](../../user/feature_flags.md), disabled by default.
+> - It's enabled on GitLab.com.
+> - It's recommended for production use.
+
+`artifacts:public` is used to determine whether the job artifacts should be
+publicly available.
+
+The default for `artifacts:public` is `true` which means that the artifacts in
+public pipelines are available for download by anonymous and guest users:
+
+```yaml
+artifacts:
+  public: true
+```
+
+To deny read access for anonymous and guest users to artifacts in public
+pipelines, set `artifacts:public` to `false`:
+
+```yaml
+artifacts:
+  public: false
+```
+
 #### `artifacts:exclude`
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/15122) in GitLab 13.1