add README for source data

_publication/SourceData/README.md

+# Source Data
+
+### Source Data Fig. 1
+Statistical source data.
+
+https://static-content.springer.com/esm/art%3A10.1038%2Fs41564-021-00941-9/MediaObjects/41564_2021_941_MOESM6_ESM.zip
+
+### Source Data Fig. 2
+Statistical source data.
+
+https://static-content.springer.com/esm/art%3A10.1038%2Fs41564-021-00941-9/MediaObjects/41564_2021_941_MOESM7_ESM.zip
+
+### Source Data Fig. 3
+Statistical source data.
+
+https://static-content.springer.com/esm/art%3A10.1038%2Fs41564-021-00941-9/MediaObjects/41564_2021_941_MOESM8_ESM.zip
+
+### Source Data Fig. 4
+Statistical source data.
+
+https://static-content.springer.com/esm/art%3A10.1038%2Fs41564-021-00941-9/MediaObjects/41564_2021_941_MOESM9_ESM.zip
+
+### Source Data Fig. 5
+Statistical source data.
+
+https://static-content.springer.com/esm/art%3A10.1038%2Fs41564-021-00941-9/MediaObjects/41564_2021_941_MOESM10_ESM.zip
+
+### Source Data Fig. 6
+Statistical source data.
+
+https://static-content.springer.com/esm/art%3A10.1038%2Fs41564-021-00941-9/MediaObjects/41564_2021_941_MOESM11_ESM.zip
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