Documentation: Add section about CPU vulnerabilities

Add documentation for the L1TF vulnerability and the mitigation mechanisms:

  - Explain the problem and risks
  - Document the mitigation mechanisms
  - Document the command line controls
  - Document the sysfs files
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lkml.kernel.org/r/20180713142323.287429944@linutronix.de

CVE-2018-3620
CVE-2018-3646

[smb: Added document as Documentation/security/l1tf.txt instead.]
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

Documentation/security/00-INDEX

@@ -24,3 +24,7 @@ tomoyo.txt
 	- documentation on the TOMOYO Linux Security Module.
 IMA-templates.txt
 	- documentation on the template management mechanism for IMA.
+l1tf.txt
+	- Describes CPU vulnerabilities and provides an overview of the
+          possible mitigations along with guidiance for selecting miti-
+          gations if they are configurable at compile, boot or run time.