Where to Start?
http://issues.apache.org/jira/browse/NIFI can be used to find tickets that are tagged as "beginner", or you can dig into any of the tickets for creating Processors. Processors should be self-contained and not rely on other outside components (except for Controller Services), so they make for excellent starting points for new NiFi developers to get started. This exposes the developer to the NiFi API and is the most extensible part of the dataflow system.
System-level and overview documentation is located in '<code checkout location>/nifi/nifi-docs/src/main/asciidoc'. Tools available to facilitate documentation generation are available at http://asciidoctor.org/docs/editing-asciidoc-with-live-preview/.