Mention New Feature in release notes

Reading a large index [has been speeded up using
pthreads](git-for-windows/git#978).

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

Author: dscho

ReleaseNotes.md

@@ -32,6 +32,12 @@ 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, msmtp, 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 v2.11.0 (December 1st 2016)
+
+### New Features
+
+* Reading a large index [has been speeded up using pthreads](https://github.com/git-for-windows/git/pull/978).
+
 ## Changes since Git for Windows v2.10.2 (November 2nd 2016)
 
 ### New Features