Add a release note (feature)

The new, experimental `git backfill` command [was
added](git-for-windows/git#5172): It helps
fetching relevant Git objects smartly in a partial, sparse clone.

Signed-off-by: gitforwindowshelper[bot] <gitforwindowshelper-bot@users.noreply.github.com>

ReleaseNotes.md

@@ -44,6 +44,7 @@ Git for Windows is distributed with other components yet, such as Bash, zlib, cu
 ### New Features
 
 * Comes with the MSYS2 runtime (Git for Windows flavor) based on [Cygwin v3.5.4](https://inbox.sourceware.org/cygwin-announce/20240825195526.2571058-1-corinna-cygwin@cygwin.de/), which drops Windows 7 and Windows 8 support.
+* The new, experimental `git backfill` command [was added](https://github.com/git-for-windows/git/pull/5172): It helps fetching relevant Git objects smartly in a partial, sparse clone.
 
 ## Changes since Git for Windows v2.46.1 (September 18th 2024)