diff --git a/upgrade_guide.md b/upgrade_guide.md
index 3cba2c9..cc57ca8 100644
--- a/upgrade_guide.md
+++ b/upgrade_guide.md
@@ -1,6 +1,7 @@
 ## References
 
 * https://docs.freebsd.org/en/books/handbook/cutting-edge/
+* https://klarasystems.com/articles/managing-boot-environments/
 
 ## Preparations