Mention New Feature in release notes

Operations in massively-sparse worktrees
[are now much faster of `core.fscache =
true`](git-for-windows/git#1344).

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

Author: dscho

ReleaseNotes.md

@@ -33,6 +33,12 @@ Git for Windows also contains Embedded CAcert Root Certificates. For more inform
 
 This package contains software from a number of other projects including Bash, zlib, curl, tcl/tk, perl, MSYS2 and a number of libraries and utilities from the GNU project, licensed under the GNU General Public License. Likewise, it contains Perl which is dual licensed under the GNU General Public License and the Artistic License.
 
+## Changes since Git for Windows v2.15.0 (October 30th 2017)
+
+### New Features
+
+* Operations in massively-sparse worktrees [are now much faster of `core.fscache = true`](https://github.com/git-for-windows/git/pull/1344).
+
 ## Changes since Git for Windows v2.14.3 (October 23rd 2017)
 
 ### New Features