update release note.

RELEASENOTE.md

@@ -1,5 +1,11 @@
-Version Notes (Current Version: v1.8)
+Version Notes (Current Version: v1.8.1)
 =======================================
+v1.8.1
+-----------
+1. Fix bug when serialize attribute argument for type array. (https://github.com/dotnet/docfx/issues/280)
+2. Fix bug when include file link to an anchor.
+3. Don't modify link when target file not existed.
+
 v1.8
 -----------
 1. Support multiple regions selection, code lines highlight and dedent length setting in [Code Snippet](http://dotnet.github.io/docfx/spec/docfx_flavored_markdown.html#code-snippet). (https://github.com/dotnet/docfx/issues/189)