update readme with instructions for google tag manager

Author: djih

README.md

@@ -391,3 +391,13 @@ You can also define the path in your RequireJS configuration like so:
     });
   </script>
 ```
+
+### Integrating with Google Tag Manager ###
+**Note: we do not officially support Google Tag Manager, but these instructions have been reported to work for people. Proceed at your own risk**
+
+1. Follow the Javascript SDK [Setup Instructions](https://github.com/amplitude/Amplitude-Javascript#setup), steps 1-3.
+2. Call SDK functions in Google Tag Manager using [Custom HTML tags](https://support.google.com/tagmanager/answer/6107167?hl=en) and adding Javascript in this form as the custom tag:
+
+```html
+<script>amplitude.logEvent('TAG');</script>
+```