Update changelog

Author: OlivierNicole

CHANGES.md

@@ -9,6 +9,7 @@
 * Compiler: Decode sourcemap mappings only when necessary (#1664)
 * Compiler: make indirect call using sequence instead of using the call method
   [f.call(null, args)] becomes [(0,f)(args)]
+* Compiler: reduce linking time by optimizing sourcemaps processing (#1617)
 * Runtime: change Sys.os_type on windows (Cygwin -> Win32)
 * Runtime: backtraces are really expensive, they need to be be explicitly
   requested at compile time (--enable with-js-error) or at startup (OCAMLRUNPARAM=b=1)