Add troubleshooting instructions about service_facts breaking Ubuntu 20.04 (#449)

* Add troubleshooting instructions about service_facts

* Put text before links

* Fix links

Author: amenasria

README.md

@@ -606,6 +606,19 @@ If you are updating from **6.14.0 or 6.14.1 on Windows**, use the following step
 
 For more details, see [Critical Bug in Uninstaller for Datadog Agent 6.14.0 and 6.14.1 on Windows][11].
 
+### Ubuntu 20.04 broken by service_facts
+
+Running the `service_facts` module on Ubuntu 20.04 causes the following error:
+
+```
+localhost | FAILED! => {
+    "changed": false,
+    "msg": "Malformed output discovered from systemd list-unit-files: accounts-daemon.service                    enabled         enabled      "
+}
+```
+
+To fix this, [update Ansible to `v2.9.8` or above][16].
+
 [1]: https://galaxy.ansible.com/Datadog/datadog
 [2]: https://github.com/DataDog/ansible-datadog
 [3]: https://docs.datadoghq.com/agent/autodiscovery
@@ -621,3 +634,4 @@ For more details, see [Critical Bug in Uninstaller for Datadog Agent 6.14.0 and
 [13]: https://github.com/DataDog/ansible-datadog/blob/main/tasks/agent-linux.yml
 [14]: https://github.com/DataDog/ansible-datadog/blob/main/tasks/agent-win.yml
 [15]: https://www.datadoghq.com/blog/datadog-marketplace/
+[16]: https://github.com/ansible/ansible/blob/stable-2.9/changelogs/CHANGELOG-v2.9.rst#id61