Add changelog

[soininen]

With this PR, we start documenting our notable changes somewhere.

No related issue.

Checklist before merging

• [ ] Documentation (also in Toolbox repo) is up-to-date
• [ ] Release notes in Toolbox repo have been updated
• [ ] Unit tests have been added/updated accordingly
• [ ] Code has been formatted by black
• [ ] Unit tests pass

[codecov-commenter]

Codecov Report

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

Comparison is base (43c1bb6) 77.14% compared to head (c874f42) 77.14%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #338 +/- ## ======================================== Coverage 77.14% 77.14% ======================================== Files 75 75 Lines 9239 9239 Branches 1949 1949 ======================================== Hits 7127 7127 Misses 1780 1780 Partials 332 332 ```

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