update docstring doc (microsoft#1645)

docs/en_US/Tutorial/Contributing.md

@@ -40,6 +40,9 @@ A person looking to contribute can take up an issue by claiming it as a comment/
 ## Code Styles & Naming Conventions
 * We follow [PEP8](https://www.python.org/dev/peps/pep-0008/) for Python code and naming conventions, do try to adhere to the same when making a pull request or making a change. One can also take the help of linters such as `flake8` or `pylint`
 * We also follow [NumPy Docstring Style](https://www.sphinx-doc.org/en/master/usage/extensions/example_numpy.html#example-numpy) for Python Docstring Conventions. During the [documentation building](Contributing.md#documentation), we use [sphinx.ext.napoleon](https://www.sphinx-doc.org/en/master/usage/extensions/napoleon.html) to generate Python API documentation from Docstring.
+* For docstrings, please refer to [numpydoc docstring guide](https://numpydoc.readthedocs.io/en/latest/format.html) and [pandas docstring guide](https://python-sprints.github.io/pandas/guide/pandas_docstring.html)
+    * For function docstring, **description**, **Parameters**, and **Returns**/**Yields** are mandatory.
+    * For class docstring, **description**, **Attributes** are mandatory.
 
 ## Documentation
 Our documentation is built with [sphinx](http://sphinx-doc.org/), supporting [Markdown](https://guides.github.com/features/mastering-markdown/) and [reStructuredText](http://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html) format. All our documentations are placed under [docs/en_US](https://github.com/Microsoft/nni/tree/master/docs).
@@ -48,4 +51,4 @@ Our documentation is built with [sphinx](http://sphinx-doc.org/), supporting [Ma
 
 * For links, please consider using __relative paths__ first. However, if the documentation is written in Markdown format, and:
     * It's an image link which needs to be formatted with embedded html grammar, please use global URL like `https://user-images.githubusercontent.com/44491713/51381727-e3d0f780-1b4f-11e9-96ab-d26b9198ba65.png`, which can be automatically generated by dragging picture onto [Github Issue](https://github.com/Microsoft/nni/issues/new) Box.
-    * It cannot be re-formatted by sphinx, such as source code, please use its global URL. For source code that links to our github repo, please use URLs rooted at `https://github.com/Microsoft/nni/tree/master/` ([mnist.py](https://github.com/Microsoft/nni/blob/master/examples/trials/mnist/mnist.py) for example).
+    * It cannot be re-formatted by sphinx, such as source code, please use its global URL. For source code that links to our github repo, please use URLs rooted at `https://github.com/Microsoft/nni/tree/master/` ([mnist.py](https://github.com/Microsoft/nni/blob/master/examples/trials/mnist/mnist.py) for example).