posted an APM doc on how to instrument future enhancements

Author: Ahil PonArul

src/APM.md

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+# Elastic APM
+
+This application has been instrumented with Elastic APM. 
+
+In order to [configure](https://www.elastic.co/guide/en/apm/agent/python/current/configuration.html) this application environment variables should be 
+used in the configMap of the `app.yaml` file.
+
+
+Custom instrumentation begins in the main method in `st_app.py` with the call
+
+```
+client = Client()
+client.begin_transaction('main_page')
+```
+and ends with `client.end_transaction('main_page')`
+
+The sidebar menu and charts are instrumented with [custom instrumentations](https://www.elastic.co/guide/en/apm/agent/python/current/api.html). 
+
+
+The `display_sidebar` has its internal functions instrumented
+with the `with` notation.
+
+The `charts` and `display_header` are instrumented with the `annotation` method.