patch 8.1.1869: code for the argument list is spread out

Problem:    Code for the argument list is spread out.
Solution:   Put argument list code in arglist.c. (Yegappan Lakshmanan,
            closes #4819)
diff --git a/src/README.md b/src/README.md
index 2e77490..ffce497 100644
--- a/src/README.md
+++ b/src/README.md
@@ -23,6 +23,7 @@
 
 File name       | Description
 --------------- | -----------
+arglist.c	| handling argument list
 autocmd.c	| autocommands
 blob.c		| blob data type
 buffer.c	| manipulating buffers (loaded files)