Add documentation for llvm-pdbutil.

llvm-svn: 310744
diff --git a/llvm/docs/CommandGuide/index.rst b/llvm/docs/CommandGuide/index.rst
index 46db57f..5a0a98c 100644
--- a/llvm/docs/CommandGuide/index.rst
+++ b/llvm/docs/CommandGuide/index.rst
@@ -51,4 +51,5 @@
    tblgen
    lit
    llvm-build
+   llvm-pdbutil
    llvm-readobj