update readme

README.md

 # Zürich bike stats
-A Renku project.
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+Exploring the bike counter data from Zürich, Switzerland.
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+The data comes from the [Zürich open data portal](https://data.stadt-zuerich.ch/).
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+Two notebooks illustrate how one can use the data:
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+* [Basic data display](notebooks/ZHBikes.ipynb)
+* [Map display of stations](notebooks/station_locations.ipynb)