docs: clarify behavior of select in confirm mapping (#621)

Relates to #557

README.md

@@ -91,6 +91,8 @@ lua <<EOF
         i = cmp.mapping.abort(),
         c = cmp.mapping.close(),
       }),
+      -- Accept currently selected item. If none selected, `select` first item.
+      -- Set `select` to `false` to only confirm explicitly selected items.
       ['<CR>'] = cmp.mapping.confirm({ select = true }),
     },
     sources = cmp.config.sources({