Documentation/admin-guide: update admin-guide index.rst

Extend index.rst index file at admin-guide root directory with
the reference to perf-security.rst file being introduced.

Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/admin-guide/index.rst b/Documentation/admin-guide/index.rst
index 965745d..0a491676 100644
--- a/Documentation/admin-guide/index.rst
+++ b/Documentation/admin-guide/index.rst
@@ -76,6 +76,7 @@
    thunderbolt
    LSM/index
    mm/index
+   perf-security
 
 .. only::  subproject and html