Update changelog

CHANGELOG.markdown

@@ -1,5 +1,6 @@
 # 2.1.0 (In development)
 
+* Backwards incompatible change: Converted the pre-cached headers to a map.
 * Converted to cowboy 2.0 (Thanks Eric, @etxemag75)
 * Deprecated `sbw:set_cookie/5` in favor of a new, more flexible,
   `sbw:set_cookie/4`, which takes an `Options` argument for specifying
@@ -12,7 +13,6 @@
 * Fix a number of yaws-related issues (max post size, keepalive)
 * Set the cowboy supervisor to not `infinity`.
 * Add a few more tests.
-* Converted the pre-cached headers to a map.
 * Ensure support up to Erlang 21.
 
 # 2.0.1