Added contributor information regarding pre-commit

[cansik]

Since it is not clear just because the file is around, I added a short chapter about how to format the code to contribute. Maybe it even makes sense to bump the isort version anyways to at least 5.12.0.

[codecov[bot]]

Codecov Report

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

Comparison is base (226d8cc) 74.80% compared to head (1e19269) 74.76%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #549 +/- ## ========================================== - Coverage 74.80% 74.76% -0.05% ========================================== Files 71 71 Lines 4695 4695 Branches 731 731 ========================================== - Hits 3512 3510 -2 - Misses 986 987 +1 - Partials 197 198 +1 ``` | [Flag](https://app.codecov.io/gh/RangiLyu/nanodet/pull/549/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/RangiLyu/nanodet/pull/549/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `74.76% <ø> (-0.05%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#carryforward-flags-in-the-pull-request-comment) to find out more.

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[cansik]

Same issue about nanodet/model/arch/one_stage_detector.py. No idea why this fails now and didn't before. I am only changing the README.md.