added link to our writing-doc document (#15756)

docs/source/add_new_model.mdx

@@ -762,7 +762,7 @@ the community to add some *Tips* to show how the model should be used. Don't hes
 regarding the docstrings.
 
 Next, make sure that the docstring added to `src/transformers/models/brand_new_bert/modeling_brand_new_bert.py` is
-correct and included all necessary inputs and outputs. It is always to good to remind oneself that documentation should
+correct and included all necessary inputs and outputs. We have a detailed guide about writing documentation and our docstring format [here](writing-documentation). It is always to good to remind oneself that documentation should
 be treated at least as carefully as the code in 🤗 Transformers since the documentation is usually the first contact
 point of the community with the model.