Mention New Feature in release notes

Comes with MSYS2 runtime (Git for Windows flavor) based on [Cygwin
2.8.1](https://cygwin.com/ml/cygwin-announce/2017-07/msg00002.html).

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

ReleaseNotes.md

@@ -38,6 +38,7 @@ This package contains software from a number of other projects including Bash, z
 ### New Features
 
 * Comes with [Git LFS v2.2.0](https://github.com/git-lfs/git-lfs/releases/tag/v2.2.0).
+* Comes with MSYS2 runtime (Git for Windows flavor) based on [Cygwin 2.8.1](https://cygwin.com/ml/cygwin-announce/2017-07/msg00002.html).
 
 ## Changes since Git for Windows v2.13.1(2) (June 15th 2017)