feat(README): Document Xdebug

Signed-off-by: Louis Chmn <louis@chmn.me>

README.md

@@ -497,6 +497,31 @@ apps = ["groupfolders"] # app to enable
 commands = ["occ groupfolders:create gf", "occ groupfolders:group 1 admin read write share delete"] # commands to run post-setup
 ```
 
+## Xdebug
+
+Haze Xdebug is running in a docker container, you need to tell you IDE how to
+properly map the path. The IDE debugger config usually looks like:
+
+```json
+	{
+		"label": "PHP: Debug server within docker",
+		"adapter": "Xdebug",
+		"request": "launch",
+		"port": 9003,
+		"pathMappings": {
+			"/var/www/html": "<sources_root_configured_in_haze.toml>",
+			"/var/www/html/apps-extra": "<app_directories_configured_in_haze.toml>",
+		},
+	},
+```
+
+This would have to be adapted for detached instances.
+
+To enable Xdebug for all requests:
+
+- Uncomment the lines in `haze <cloud-id> edit /config/php.ini`
+- Then run `haze reload <cloud-id>`
+
 ## FrankenPHP (experimental)
 
 You can have Nextcloud run on FrankenPHP.