[DOC] Provide an example when no framework is used (#680)

* [DOC] Provide an example when no framework is used

Provide an example when no framework is used.
Feel free to edit or enrich with additional references to other documentation pages.

Author: lucabelluccini

docs/how-the-agent-works.asciidoc

@@ -26,6 +26,41 @@ Examples of this are
 Framework integrations require some limited code changes in your app.
 E.g. for Django, you need to add `elasticapm.contrib.django` to `INSTALLED_APPS`.
 
+[float]
+[[how-it-works-no-framework]]
+==== What if you are not using a framework
+
+If you're not using a supported framework, for example, a simple Python script, you can still
+leverage the agent's <<automatic-instrumentation,automatic instrumentation>>.
+
+In order to collect the Spans generated by the supported libraries, you need to invoke `elasticapm.instrument()`
+(just once, at the initalization stage of your application) and create at least one Transaction.
+It is up to you to determine what you consider a Transaction within your application -- it can be the whole execution of the
+script or a part of it.
+
+The example below will consider the whole execution as a single transaction with two HTTP request Spans in it.
+The `elasticapm.Client` can be setup programmatically or using the environment variables.
+
+[source,python]
+----
+import requests
+import time
+import elasticapm
+
+def main():
+    sess = requests.Session()
+    for url in [ 'https://www.elastic.co', 'https://benchmarks.elastic.co' ]:
+        resp = sess.get(url)
+        time.sleep(1)
+
+if __name__ == '__main__':
+    client = elasticapm.Client()
+    elasticapm.instrument()
+    client.begin_transaction('main')
+    main()
+    client.end_transaction('main')
+----
+
 [float]
 [[how-it-works-instrumentation]]
 ==== Instrumentation
@@ -61,4 +96,4 @@ the agent starts two additional threads per process:
 
 Note that every process that instantiates the agent will have these three threads.
 This means that when you e.g. use gunicorn or uwsgi workers,
-each worker will have three threads started by the Python agent.
+each worker will have three threads started by the Python agent.