doc: ReSTify and split LSM.txt

The existing LSM.txt file covered both usage and development, so split
this into two files, one under admin-guide and one under kernel
development.

Cc: James Morris <james.l.morris@oracle.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/security/LSM.rst

@@ -0,0 +1,14 @@
+=================================
+Linux Security Module Development
+=================================
+
+Based on https://lkml.org/lkml/2007/10/26/215,
+a new LSM is accepted into the kernel when its intent (a description of
+what it tries to protect against and in what cases one would expect to
+use it) has been appropriately documented in ``Documentation/security/LSM``.
+This allows an LSM's code to be easily compared to its goals, and so
+that end users and distros can make a more informed decision about which
+LSMs suit their requirements.
+
+For extensive documentation on the available LSM hook interfaces, please
+see ``include/linux/lsm_hooks.h``.