add statsbomb readme

data/fb_big5_advanced_statsbomb/README.md

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+# StatsBomb via FBRef
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+FBRef changed data providers from StatsBomb to Opta late October 2022. This meant that all previously displayed data changed overnight to reflect Opta's counting/estimating of statistics.
+
+For any analysts looking to maintain previous analysis, or to be able to compare StatsBomb and Opta, use the data files in this directory for StatsBomb data, which was last updated 2022-08-22.
+
+For the equivalent Opta data sets, see the `data/fb_big5_advanced_season_stats` directory in this repository.