Mention New Feature in release notes

There are now [experimental built-in versions of `git add -i` and
`git add -p`](git-for-windows/git#2150),
i.e. those modes are now a lot faster (in particular at startup). You
can opt into using them on the last installer page.

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

Author: dscho

ReleaseNotes.md

@@ -48,6 +48,7 @@ This package contains software from a number of other projects including Bash, z
 * Comes with [OpenSSH v8.0p1](https://www.openssh.com/txt/release-8.0).
 * Comes with [Git LFS v2.7.2](https://github.com/git-lfs/git-lfs/releases/tag/v2.7.2).
 * Comes with MSYS2 runtime (Git for Windows flavor) based on [Cygwin 3.0.7](https://cygwin.com/ml/cygwin-announce/2019-04/msg00030.html).
+* There are now [experimental built-in versions of `git add -i` and `git add -p`](https://github.com/git-for-windows/git/pull/2150), i.e. those modes are now a lot faster (in particular at startup). You can opt into using them on the last installer page.
 
 ### Bug Fixes