Update README.md with "maintained by Plotly" badge

Author: gvwilson

README.md

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 # Plotly Sample Datasets
 
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 | Dataset name | Best for | Origin | Chart example | Download | Open in Plotly |
 | --- | --- | --- | ---  | ---  | ---  |
 | Walmart store openings | Maps | Unknown | [Python](https://plot.ly/python/map-subplots-and-small-multiples/) | [Download](https://github.com/plotly/datasets/blob/master/1962_2006_walmart_store_openings.csv) | [Open](https://plot.ly/16263/~jackp/) |