add instructions for creating docstrings

docs/source/contributing.mdx

@@ -10,7 +10,9 @@ Now all the pre-commit hooks will be automatically run when you try to commit an
 
 ## Doc-string syntax
 
-TODO: Add description + reference of HF docstring best practices.
+We're following NumPy doc-string conventions with the only notable difference being that we use Markdown instead of Rich text format (RTF) for markup within the doc-strings.
+
+Please see the existing documentation to see how to generate autodocs.
 
 ## Documentation
 - [guideline for documentation syntax](https://github.com/huggingface/doc-builder#readme)