Add CircleCI to the Well Known Annotations (#3082)

docs/features/software-catalog/well-known-annotations.md

@@ -158,6 +158,27 @@ fallback (`rollbar.organization` followed by `organization.name`).
 Specifying this annotation may enable Rollbar related features in Backstage for
 that entity.
 
+### circleci.com/project-slug
+
+```yaml
+# Example:
+metadata:
+  annotations:
+    circleci.com/project-slug: github/spotify/pump-station
+```
+
+The value of this annotation is the so-called slug (or alternatively, the ID) of
+a [CircleCI](https://circleci.com/) project within your organization. The value
+can be the format of `[source-control-manager]/[organization]/[project-slug]` or
+just `[organization]/[project-slug]`. When the `[source-control-manager]` slug
+is omitted, `bitbucket` will be used as a fallback.
+
+Specifying this annotation will cause the CI/CD features in Backstage to display
+data from CircleCI for that entity.
+
+Providing both the `github.com/project-slug` and `circleci.com/project-slug`
+annotations can cause problems as both may be used for CI/CD features.
+
 ### backstage.io/ldap-rdn, backstage.io/ldap-uuid, backstage.io/ldap-dn
 
 ```yaml