Merge branch 'readme' into 'main'

add readme.

See merge request cepc/CEPCSWData!1

README.md

+# CEPCSW Data
+
+`git lfs` is used to keep track of these binary data. 
+
+Please make sure initialize `git lfs` first:
+```bash
+$ git lfs install
+> Git LFS initialized.
+```