Add a cookbook that enables floweaver to handle the path-visualization of highlighted-node (and combines floweaver with ipywidgets)

docs/cookbook/floweaver-path/README.md

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+# floweaver-path example
+
+![Demo](https://github.com/fullflu/floweaver-path/blob/master/demo/floweaver_path_demo.gif)
+
+I implement [floweaver-path](https://github.com/fullflu/floweaver-path), an extension of the [floweaver](https://sankeyview.readthedocs.io/en/latest/) to handle the visualization of paths that pass through a selected node.
+
+It would be reasonable to maintain floweaver-path apart from the original floweaver at the moment.
+Therefore, I describe only the summary of the extension and put the minimum resources (`data/template.csv` and `template_before_separation.ipynb`) here.
+
+Please check my repository above if you want to know the details of how floweaver-path works.
+
+If you have any questions, please feel free to ask me ([@fullflu](https://github.com/fullflu)).
+
+## Summary
+We focus on the visualization of longitudinal data.
+
+The idea of our visualization is based on [pathSankey](https://bl.ocks.org/jeinarsson/e37aa55c3b0e11ae6fa1), that is an extension of [d3-sankey](https://github.com/d3/d3-sankey).
+
+The color of paths that pass through a selected node is yellow-green (highlighted), and that of other paths is gray.
+
+You can interactively select a node by using dropdowns in jupyter notebook.
+
+We have two technical contributions to the field of visualization using Sankey diagrams.
+
+One is to extend the layer number:
+- Ordinary Sankey diagrams can only visualize paths between 2 layers.
+- pathSankey can only visualize paths between 3 layers.
+- We can visualize the comparison of paths between two layers before and after (up to 5 layers).
+
+The other is to create a notebook that can interact with users. We integrate several functions of ipywidgets into floweaver.
+
+## Requirement
+
+### in this repository (using pip or conda)
+- floweaver (==2.0.0a5)
+- ipysankeywidget (==0.2.5)
+
+### in floweaver-path repository (using docker)
+- docker (installs two libraries: floweaver, ipysankeywidget)
+
+### in both repositories
+- input file (`*.csv, *.pickle or *.xlsx` should be put in `data` directory)
+
+
+## Structure
+```
+├── README.md
+├── data
+│   └── template.csv
+└── template_before_separation.ipynb
+```