Update changelog for 0.22.1

Author: Edward Thomson

CHANGES.md

@@ -10,6 +10,12 @@
   - Windows (x86/amd64): <https://ci.appveyor.com/project/libgit2/libgit2sharp>
   - Linux/Mac OS X: <https://travis-ci.org/libgit2/libgit2sharp>
 
+## v0.22.1 - ([diff](https://github.com/libgit2/libgit2sharp/compare/v0.22...v0.22.1))
+
+### Fixes
+
+ - Update to libgit2 0.25.1 which includes security fixes for smart protocol parsing and TLS certificate validity check callbacks.
+
 ## v0.22 - ([diff](https://github.com/libgit2/libgit2sharp/compare/v0.21.1...v0.22))
 
 ### Additions