Add documentation for model dispatch

Author: rlouf

docs/reference/dispatch.md

@@ -1 +1,24 @@
 # Model-based prompt dispatching
+
+
+Different models often require different prompts to achieve a given task. They are, in essence, not different prompts in the sense that they are supposed to perform the same operation. In the same way we use `functools.singledispatch` to dispatch a functionality on the type of the first
+argument, it can be useful to dispatch the prompt on the model that is being used.
+
+`prompts` provides a way to dispatch the prompt on the model:
+
+
+```python
+import prompts
+
+
+@prompts.template
+def a_simple_prompt(query: str):
+    """<s>{{ query }}</s>"""
+
+@a_simple_prompt.register("google/gemma-2-9b")
+def a_simple_prompt_gemma(query: str):
+    """<bos>{{ query }}<eos>"""
+```
+
+!!! note
+    Choosing BOS and EOS based on the model is better achieved by using [special variables](special_tokens.md).