change hardcoded master branch in readme enumeration to HEAD

diff --git a/RelNotes b/RelNotes
index e7fe59f5d0dabb65fa73e558f937486f229773f0..fc657e7d2f4b22ce71b2983bd33d9fca131c0c37 120000 (symlink)
--- a/RelNotes
+++ b/RelNotes
@@ -1 +1 @@
-Documentation/RelNotes/2.15.3.txt
\ No newline at end of file
+Documentation/RelNotes/2.24.0.txt
\ No newline at end of file