Mention New Feature in release notes

The FSCache feature [was further optimized in particular for very
large repositories](git-for-windows/git#1937).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

ReleaseNotes.md

@@ -38,6 +38,7 @@ This package contains software from a number of other projects including Bash, z
 ### New Features
 
 * Comes with [OpenSSL v1.1.1a](https://www.openssl.org/news/openssl-1.1.1-notes.html). The OpenSSH and cURL packages were rebuilt to make use of OpenSSL v1.1.1a.
+* The FSCache feature [was further optimized in particular for very large repositories](https://github.com/git-for-windows/git/pull/1937).
 
 ### Bug Fixes