Add WARNING message

Author: lambdalisue

README.md

@@ -6,3 +6,7 @@ To test denops plugin startup time with massive plugins.
 
 Clone the repository and generate plugins with `./generate.sh`.
 Add the working tree to `runtimepath`in your`vimrc`.
+
+> [!WARNING]
+> Do NOT forget to remove this repository from `runtimepath` after testing.
+> At least for denops v5 or earlier, it will cause a slowdown in startup time.