Update Block Explorer README (#2733)

## Motivation

This README was just showing how to build the Block Explorer code

## Proposal

Include also instructions on how to serve the web application and open it in your browser

## Test Plan

Went through the steps myself

## Release Plan

- Nothing to do / These changes follow the usual release cycle.

Author: Andre da Silva

linera-explorer/README.md

@@ -12,10 +12,17 @@ This module provides web files to run a block explorer from Linera service node
 npm install
 ```
 and then
-```
+```bash
 npm run full
 ```
 
+Then, to serve the web application on a local server, run:
+```bash
+npm run serve
+```
+
+The URL to access the Block Explorer will be printed.
+
 ## Contributing
 
 See the [CONTRIBUTING](../CONTRIBUTING.md) file for how to help out.