CHANGELOG: mention blobstore change (#220)

Signed-off-by: Stephen Gutekanst <[email protected]>

Author: Stephen Gutekanst

charts/sourcegraph/CHANGELOG.md

@@ -8,6 +8,9 @@ Use `**BREAKING**:` to denote a breaking change
 
 ## Unreleased
 
+## 4.2.1
+
+* `minio` has been replaced with `blobstore`. If you use Sourcegraph's built-in object storage (instead of an external S3/GCP bucket), then please see the update notes here: https://docs.sourcegraph.com/admin/how-to/blobstore_update_notes
 
 ## 4.2.0