add a pre-commit badge to README

[valeriupredoi]

Description

It's a good idea to have one! Badge code is visible on our Pre-Commit page and can be seen in action on the branch https://github.com/ESMValGroup/ESMValCore/tree/add_precommit_badge

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Before you get started

• [ ] ☝ Create an issue to discuss what you are going to do

Checklist

It is the responsibility of the author to make sure the pull request is ready to review. The icons indicate whether the item will be subject to the 🛠 Technical or 🧪 Scientific review.

• [x] 🛠 This pull request has a descriptive title and labels
• [x] 🛠 All checks below this pull request were successful

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To help with the number pull requests:

• 🙏 We kindly ask you to review two other open pull requests in this repository

[codecov[bot]]

Codecov Report

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

Project coverage is 94.83%. Comparing base (4b0dd41) to head (e75261c). Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2534 +/- ## ======================================= Coverage 94.83% 94.83% ======================================= Files 251 251 Lines 14191 14191 ======================================= Hits 13458 13458 Misses 733 733 ```

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