Mention Bug Fix in release notes

When building with MS Visual C, [release builds are now properly
optimized](git-for-windows/git#948).

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

Author: dscho

ReleaseNotes.md

@@ -40,6 +40,7 @@ This package contains software from a number of other projects including Bash, z
 * When encountering a symbolic link, Git [now always tries to read it](https://github.com/git-for-windows/git/issues/958), not only when `core.symlinks = true`.
 * The regression where Git would not interpret non-ASCII characters passed from a CMD window correctly [has been fixed](https://github.com/git-for-windows/git/issues/945).
 * Performance of the cache of case-insensitive file names [has been improved](https://github.com/git-for-windows/git/pull/964).
+* When building with MS Visual C, [release builds are now properly optimized](https://github.com/git-for-windows/git/pull/948).
 
 ## Changes since Git for Windows v2.10.1(2) (October 13th 2016)