Add an extra script in `Contributing` to install some of the extra `Doc` requirements i.e. to make it easier to dev & QA

Using these commands from Contributing

(plotly_dev) $ pip install -r packages/python/plotly/requirements.txt
(plotly_dev) $ pip install -r packages/python/plotly/optional-requirements.txt

won't install modules e.g. wget, datashader, etc. that are used in docs.
I suggest we add optional-doc-requirements.txt to help install some of these modules so that we could more efficiently test the examples using:

jupyter-notebook doc/python

从 contributing.md 的“Install requirements — non-windows”部分以及现有的 packages/python/plotly/requirements.txt 和 optional-requirements.txt 文件开始。检查 doc/python 下的文档示例，并确定其中使用的额外模块，包括 wget 和 datashader。完成标准是：提供 optional-doc-requirements.txt 文件并更新贡献者说明，使这些示例可以通过 jupyter-notebook doc/python 进行测试。