Documentation: clarify the purpose of LSMs

Clarify the purpose of the LSM interface with some brief examples and
pointers to additional documentation.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: James Morris <jmorris@namei.org>
diff --git a/Documentation/security/00-INDEX b/Documentation/security/00-INDEX
index 19bc494..99b85d3 100644
--- a/Documentation/security/00-INDEX
+++ b/Documentation/security/00-INDEX
@@ -1,5 +1,7 @@
 00-INDEX
 	- this file.
+LSM.txt
+	- description of the Linux Security Module framework.
 SELinux.txt
 	- how to get started with the SELinux security enhancement.
 Smack.txt