Add environment sample for OpenStack Swift (#4816)

.env.production.sample

@@ -98,6 +98,15 @@ SMTP_FROM_ADDRESS=notifications@example.com
 # S3_ENDPOINT=
 # S3_SIGNATURE_VERSION=
 
+# Swift (optional)
+# SWIFT_ENABLED=true
+# SWIFT_USERNAME=
+# SWIFT_TENANT=
+# SWIFT_PASSWORD=
+# SWIFT_AUTH_URL=
+# SWIFT_CONTAINER=
+# SWIFT_OBJECT_URL=
+
 # Optional alias for S3 if you want to use Cloudfront or Cloudflare in front
 # S3_CLOUDFRONT_HOST=