Fix NEWS for 5.6.0 and 5.6.1.

NEWS

@@ -4,6 +4,9 @@ XZ Utils Release Notes
 
 5.6.1 (2024-03-09)
 
+    IMPORTANT: This fixed bugs in the backdoor (CVE-2024-3094) (someone
+    had forgot to run Valgrind).
+
     * liblzma: Fixed two bugs relating to GNU indirect function (IFUNC)
       with GCC. The more serious bug caused a program linked with
       liblzma to crash on start up if the flag -fprofile-generate was
@@ -30,6 +33,9 @@ XZ Utils Release Notes
 
 5.6.0 (2024-02-24)
 
+    IMPORTANT: This added a backdoor (CVE-2024-3094). It's enabled only
+    in the release tarballs.
+
     This bumps the minor version of liblzma because new features were
     added. The API and ABI are still backward compatible with liblzma
     5.4.x and 5.2.x and 5.0.x.