Update the README to include a link to the Polkadot SDK Version Manag…

…er (#4718)

Adds a link to the [Polkadot SDK Version
Manager](https://github.com/paritytech/psvm) since this tool is not well
known, but very useful for developers using the Polkadot SDK.

---------

Co-authored-by: Bastian Köcher <git@kchr.de>

README.md

@@ -81,3 +81,7 @@ fellowship, this separation, the RFC process
 This repository is the amalgamation of 3 separate repositories that used to make up Polkadot SDK,
 namely Substrate, Polkadot and Cumulus. Read more about the merge and its history
 [here](https://polkadot-public.notion.site/Polkadot-SDK-FAQ-fbc4cecc2c46443fb37b9eeec2f0d85f).
+
+## Other useful resources and tooling
+
+* A simple tool to manage and update the Polkadot SDK dependencies (https://github.com/paritytech/psvm)