Update README with detailed Xdebug debugging instructions

This commit enhances the debugging section of the README with vital information about:
- The correct path mapping configuration required between host and container
- Clear explanation of why the mapping is crucial for breakpoints to work
- Step-by-step instructions for enabling and using the debugger
- Troubleshooting tips for common Xdebug issues

This documentation will help developers avoid common pitfalls when setting up
Xdebug with VSCode in a Docker environment, particularly regarding the critical
path mapping between the local ./web directory and the container's /var/www/html path.

Author: nanoninja

README.md

@@ -174,7 +174,7 @@ VS Code requires the PHP Debug extension to work with Xdebug. Here's how to set
             "request": "launch",
             "port": 9003,
             "pathMappings": {
-                "/var/www/html": "${workspaceFolder}"
+                "/var/www/html": "${workspaceFolder}/web"
             },
             "log": true,
             "xdebugSettings": {
@@ -188,6 +188,18 @@ VS Code requires the PHP Debug extension to work with Xdebug. Here's how to set
 }
 ```
 
+When using Xdebug with Docker and VSCode, it's crucial to correctly configure the path mapping between your local filesystem and the Docker container's filesystem.
+
+In our configuration, the local directory `./web` is mounted to the path `/var/www/html` in the container. This distinction is essential for understanding how to configure VSCode.
+
+Your `launch.json` file in VSCode should contain the following mapping:
+
+```json
+"pathMappings": {
+    "/var/www/html": "${workspaceFolder}/web"
+}
+```
+
 3. **Start debugging**
    - Set breakpoints in your code by clicking in the gutter next to line numbers
    - Start the debugging session in VS Code (F5 or click the green play button in the Debug panel)