Mention New Feature in release notes

The FSCache feature (which was labeled experimental
for quite some time) [is now enabled by
default](#101).

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

installer/ReleaseNotes.md

@@ -34,6 +34,10 @@ This package contains software from a number of other projects including Bash, z
 
 ## Changes since Git for Windows v2.7.4 (March 18th 2016)
 
+### New Features
+
+* The FSCache feature (which was labeled experimental for quite some time) [is now enabled by default](https://github.com/git-for-windows/build-extra/pull/101).
+
 ### Bug Fixes
 
 * The previous workaround for the blurred link to the Git Credential Manager [was fixed](https://github.com/git-for-windows/build-extra/commit/58d978cb84096bcc887170cbfcf44af022848ae3) so that the link is neither blurry nor overlapping.