Update default_behavior

README.md

@@ -63,7 +63,7 @@ lua <<EOF
       ['<C-Space>'] = cmp.mapping.complete(),
       ['<C-e>'] = cmp.mapping.close(),
       ['<CR>'] = cmp.mapping.confirm({
-        behavior = cmp.ConfirmBehavior.Replace,
+        behavior = cmp.ConfirmBehavior.Insert,
         select = true,
       })
     },
@@ -261,6 +261,10 @@ cmp.setup {
 }
 ```
 
+#### nvim-cmp is slow.
+
+I optimized nvim-cmp as much as possible but some reason exists maybe.
+
 
 Source creation
 ====================