add developer guide for loading data

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 48.76%. Comparing base (b8d5fd8) to head (e5f8f85). Report is 6 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #74 +/- ## ======================================= Coverage 48.76% 48.76% ======================================= Files 4 4 Lines 121 121 ======================================= Hits 59 59 Misses 62 62 ```

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