Add changelog for transactions

CHANGELOG.md

@@ -8,6 +8,11 @@ v0.21 + 1
 
 * Use a map for the treebuilder, making insertion O(1)
 
+* Introduce reference transactions, which allow multiple references to
+  be locked at the same time and updates be queued. This also allows
+  us to safely update a reflog with arbitrary contents, as we need to
+  do for stash.
+
 * LF -> CRLF filter refuses to handle mixed-EOL files
 
 * LF -> CRLF filter now runs when * text = auto (with Git for Windows 1.9.4)