Merge branch 'patch-developer-note' into 'stable-3.1'

Add README to indicate where to override these templates

See merge request ispconfig/ispconfig3!848

install/tpl/README.md

+Overriding templates
+--------------------
+
+Manually altered versions have to be put into `server/conf-custom/install/`, e.g. `/usr/local/ispconfig/server/conf-custom/install/`