chore: multichain-node update README.md and add .nvmrc (#1367)

* chore: update multichain-node README.md + add .nvmrc

Author: ffmcgee725

playground/multichain-node/.nvmrc

@@ -0,0 +1 @@
+v20.19.0

playground/multichain-node/README.md

@@ -2,6 +2,13 @@
 
 Get started by running:
 
+```bash
+# This repo expects you to use `nvm` (https://github.com/nvm-sh/nvm) for node version management.
+# if that's not the case, make sure to ignore this bash command,
+# and manually set your node to v20.19.0 or above.
+nvm use
+```
+
 ```bash
 yarn install
 ```