Add a CHANGELOG entry for the filebuf change
diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
index 0000000..46b0759
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,5 @@
+v0.21 + 1
+------
+
+* File unlocks are atomic again via rename. Read-only files on Windows are
+ made read-write if necessary.