Document Plugin System

[kumavis]

references
https://github.com/obsidianforensics/hindsight/blob/894dad7240a6028a90c5f41b11486b67b30da0ee/pyhindsight/analysis.py#L571
https://github.com/obsidianforensics/hindsight/tree/894dad7240a6028a90c5f41b11486b67b30da0ee/pyhindsight/plugins