WIP add datasets

Author: ogrisel

content/datasets/README.md

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+## Electricity load data (France)
+
+The data is manually retrieved from the ENTSOE Open Data portal. The CSV file needs to be downloaded from an browser by clicking for the right fields in a form:
+
+https://transparency.entsoe.eu/load-domain/r2/totalLoadR2/show?name=&defaultValue=false&viewType=TABLE&areaType=BZN&atch=false&dateTime.dateTime=12.10.2023+00:00|UTC|DAY&biddingZone.values=CTY|10YFR-RTE------C!BZN|10YFR-RTE------C&dateTime.timezone=UTC&dateTime.timezone_input=UTC#
+
+- click the "Export Data" menu, then login / create and account;
+- once logged-in, navigate back to the link above;
+- make sure to select UTC time;
+- tick the country of interest ("BZN|FR" in our case) in the left-hand panel;
+- put a date within the year of interest (between 2020 and now);
+- click the down arrow of the "Export Data" menu, and select "Total Load - Day
+  Ahead / Actual (Year, CSV)".
+
+## Weather data:
+
+From https://open-meteo.com/en/docs/historical-forecast-api. The data is
+fetched from the `fetch_weather_data.py` script.