add explanatory note

Author: khxu

data/eu_dsa/README.md

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 # Average Monthly Active Users for EU
 
 The Digital Services Act (DSA) requires online platforms to report information on the average monthly active users (MAU) in the EU, defined as the number of users in EU member states that either: (1) are exposed to content hosted by the online platform and disseminated through the platform’s online interface; or (2) request that the online platform host content. The average MAU for a given month is based on the average of the MAU of the preceding 6 months, so for example, the average MAU for January 2024 is calculated by taking the average of the MAU of July 2023 through December 2023.
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+## Errata
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+- 2024-04-23: We discovered an error in our process for calculating EU MAU for npm, which resulted in us understating the average MAU in previous reporting periods. This error has now been corrected.