add notes on why some data files have been dropped

Ecal/data/README.md

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+# ECal Data Files
+Most of the data files are related to various BDTs
+(Boosted Decision Trees) that were used while studying the ECal's
+veto performance. Only two of the files have been
+copied from the legacy submodule into ldmx-sw v4.
+
+You can still download the legacy data files from
+the archived submodule [LDMX-Software/Ecal](https://github.com/LDMX-Software/Ecal).
+Putting them in this directory will mean they will
+be installed along side the current BDT model (gabrielle)
+and the cell locations text file (cellxy).