runtime(doc): include vietnamese.txt

Since Vietnamese keymaps in Vim is quite differences from the
corresponding input methods, let's document the Vietnamese specifics in
vietnames.txt

related: #16144

Signed-off-by: Phạm Bình An <111893501+brianhuster@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/Makefile b/runtime/doc/Makefile
index 707b24b..00ea541 100644
--- a/runtime/doc/Makefile
+++ b/runtime/doc/Makefile
@@ -151,6 +151,9 @@
 os_win32.txt:
 	touch $@
 
+vietnamese.txt:
+	touch $@
+
 # In *BSD, the variable '$<' is used in suffix-transformation rules (in GNU this
 # is called "implicit rules", and in MS Windows it is called "inference rules").
 # For code portability, it is better to use the '$?' variable for explicit rules.