Add pre-commit badge to README.md

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 29.41176% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 27.27%. Comparing base (3fce6e2) to head (b31b10e).

Files	Patch %	Lines
src/molara/Structure/io/importer_crystal.py	25.00%	3 Missing :warning:
src/molara/Gui/crystal_dialog.py	0.00%	2 Missing :warning:
src/molara/Gui/main_window.py	0.00%	2 Missing :warning:
src/molara/Gui/structure_widget.py	0.00%	2 Missing :warning:
src/molara/Gui/supercell_dialog.py	0.00%	2 Missing :warning:
src/molara/Gui/measuring_tool_dialog.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #321 +/- ## ========================================== - Coverage 27.28% 27.27% -0.02% ========================================== Files 37 37 Lines 3126 3124 -2 ========================================== - Hits 853 852 -1 + Misses 2273 2272 -1 ```

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[ab5424]

@adrianusler There are a few errors in the pre-commit, do you have a quick fix for them?

src/molara/Gui/supercell_dialog.py:75: error: Unsupported target for indexed assignment ("Sequence[int]")  [index]
src/molara/Gui/supercell_dialog.py:76: error: Unsupported target for indexed assignment ("Sequence[int]")  [index]
src/molara/Gui/supercell_dialog.py:77: error: Unsupported target for indexed assignment ("Sequence[int]")  [index]
src/molara/Gui/structure_widget.py:354: error: "Structure" has no attribute "basis_vectors"  [attr-defined]
src/molara/Gui/measuring_tool_dialog.py:66: error: List item 0 has incompatible type "None"; expected "str"  [list-item]
src/molara/Gui/main_window.py:180: error: Incompatible return value type (got "None", expected "bool")  [return-value]