Add the .cact format guide

README.md

@@ -90,6 +90,7 @@ Every turn returns one JSON object with `function_calls`, the model's `reasoning
 - [Fine-tuning Needle](https://cactuscompute.com/blog/finetuning-needle): the data format, the commands, reading the loss, sizing the dataset.
 - [Needle Python docs](https://cactuscompute.com/blog/needle-python-docs): the API, the response shape, the behaviour contract, system facts, tool retrieval, offline devices, environments, the CLI.
 - [What devices are supported on Needle](https://cactuscompute.com/blog/needle-supported-devices): every platform folder, the CLI runner, the C API, the browser, WASI, air-gapped setup.
+- [The .cact format](https://cactuscompute.com/blog/cact-format): the file the engine maps and reads in place, Cactus Quants at 2.125 bits per weight, and how to parse it yourself.
 
 ## Customisation