Update digest_upgrade.md

docs/_rd/digest_upgrade.md

@@ -70,4 +70,8 @@ Communication::Block.all.find_each { |block|
   crawl(block.data)
   block.save
 }
-```
\ No newline at end of file
+```
+
+## Sources
+
+https://www.bigbinary.com/blog/how-we-upgraded-from-rails-6-to-rails-7