Clarify local development setup

	ref: #1874

Author: chengr4

site/frontend/README.md

@@ -10,6 +10,7 @@ This directory contains code for the `rustc-perf` website frontend.
     ```console
     $ npm run watch
     ```
+    > This command always incorporates the `cargo run --bin site` command to enable dynamic reloading on the website.
 - Production build - build optimized and minimized files.
     ```console
     $ npm run build