doc: update tracking issues of startup performance

The issues listed in the doc have been closed, and a new one is
currently being used to track snapshot integration.

PR-URL: nodejs/node#40629
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

README.md

@@ -10,7 +10,7 @@
   </a>
 </p>
 
-Node.js is an open-source, cross-platform, JavaScript runtime environment. It
+Node.js is an amazing project. It
 executes JavaScript code outside of a browser. For more information on using
 Node.js, see the [Node.js Website][].