added howto file

HOWTOS.md

+Howto's
+=======
+
+Modify main NiFi pipeline
+-------------------------
+
+To make modifications to the main NiFi pipeline and add it to the Ansible playbook, do the following in the soctool directory:
+
+* Make necesarry to the pipeline in the NiFi GUI
+* Copy flow.xml.gz file from one of the NiFi containers:  
+  `docker cp <CONTAINER ID>:/opt/nifi/nifi-current/conf/flow.xml.gz .`
+* Convert flowx.xml.gz to new template  
+  `utils/flow2template.py flow.xml.gz roles/nifi/templates/flow.xml.j2`