added section on updating the json file

README.md

@@ -52,4 +52,7 @@ Currently files can be read from these three locations:
 beambackgrounds = get_beam_background_by_key("experiment22_run30")
 add_simulation(basf2_path, bkgfiles=beambackgrouns.gridka_file_list)
 
-```
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+```
+
+### Editing the JSON file
+To update entries in the json file or add new entries, you simply create an `BeambackgroundMetadata` with the desired values. Then call its `update_json_entry` function. Be aware, that this will update the JSON file for everybody, so update with care. If you want to have a special version of a certain entry, create a new entry by setting the `key` property.
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