[LFI] Add LFI to UserGuides toctree

Author: zyedidia

llvm/docs/UserGuides.rst

@@ -49,6 +49,7 @@ intermediate LLVM representation.
    InstrProfileFormat
    InstrRefDebugInfo
    KeyInstructionsDebugInfo
+   LFI
    LinkTimeOptimization
    LoopTerminology
    MarkdownQuickstartTemplate
@@ -309,3 +310,6 @@ Additional Topics
 
 :doc:`Telemetry`
    This document describes the Telemetry framework in LLVM.
+
+:doc:`LFI <LFI>`
+    This document describes the Lightweight Fault Isolation (LFI) target in LLVM.