Add source option to examples in README (#353)

Author: Mehdi-1komma5grad

README.md

@@ -72,6 +72,7 @@ app.use(
       gtm_preview: 'env-4',
       gtm_cookies_win: 'x',
     },
+    source: 'https://customurl.com/gtm.js', // Add your own serverside GTM script
     defer: false, // Script can be set to `defer` to speed up page load at the cost of less accurate results (in case visitor leaves before script is loaded, which is unlikely but possible). Defaults to false, so the script is loaded `async` by default
     compatibility: false, // Will add `async` and `defer` to the script tag to not block requests for old browsers that do not support `async`
     nonce: '2726c7f26c', // Will add `nonce` to the script tag