docs: mention Diátaxis in README; drop completed TODO item

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dfa1

README.md

@@ -115,6 +115,8 @@ java -jar cli/target/vortex.jar <subcommand> [args]
 
 ## Documentation
 
+Docs follow the [Diátaxis](https://diataxis.fr/) framework (tutorial, how-to, reference, explanation).
+
 | Document | Contents |
 |---|---|
 | [docs/tutorial.md](docs/tutorial.md) | Step-by-step: write and read your first Vortex file |

TODO.md

@@ -53,7 +53,6 @@
 
 ## Documentation
 
-- [ ] Use Diátaxis (https://diataxis.fr/) to structure docs: tutorials, how-to guides, reference, explanation
 - [ ] Format specification: byte-exact diagrams for file layout and each encoding, with annotated examples (Arrow spec style)
 - [ ] how to use the library and the cli