docs: add citation section

README.md

@@ -419,3 +419,15 @@ only; nothing was ever tuned on an eval set; every negative result is published.
 
 *Built by one person and an AI assistant in about a week of evenings, for about
 the price of a game console. The failures are the useful part.*
+
+## Citation
+
+```bibtex
+@misc{saravanan2026thimble,
+  title  = {Thimble: A 48M-Parameter Tool-Calling Model from Co-Designing the
+            Tokenizer, Loss, and Decoder --- with Eleven Negative Results},
+  author = {Saravanan, Nikshep},
+  year   = {2026},
+  url    = {https://github.com/nikshepsvn/thimble}
+}
+```