docs(readme): add information for upgrading to v4.x

[FlorianSDV]

INT-696

[codecov[bot]]

Codecov Report

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

Project coverage is 48.15%. Comparing base (5c291a9) to head (91f1f18). Report is 11 commits behind head on main.

:white_check_mark: All tests successful. No failed tests found.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #284 +/- ## ============================================ - Coverage 51.96% 48.15% -3.81% + Complexity 692 561 -131 ============================================ Files 152 148 -4 Lines 4191 3721 -470 Branches 55 55 ============================================ - Hits 2178 1792 -386 + Misses 1958 1874 -84 Partials 55 55 ``` | [Flag](https://app.codecov.io/gh/myparcelnl/prestashop/pull/284/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=myparcelnl) | Coverage Δ | | |---|---|---| | [backend](https://app.codecov.io/gh/myparcelnl/prestashop/pull/284/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=myparcelnl) | `76.97% <ø> (-1.07%)` | :arrow_down: | | [frontend](https://app.codecov.io/gh/myparcelnl/prestashop/pull/284/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=myparcelnl) | `0.00% <ø> (ø)` | | 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=myparcelnl#carryforward-flags-in-the-pull-request-comment) to find out more.

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[myparcel-bot[bot]]

✅ Build successful!

You can find a link to the downloadable artifact below.

Name	Link
Commit	50e8bcf7eb9eaf89e36b64a986673ac88bff08ac
Logs	https://github.com/myparcelnl/prestashop/actions/runs/11599135807
Download	https://github.com/myparcelnl/prestashop/suites/30300843986/artifacts/2124647595

[myparcel-bot[bot]]

:tada: This PR is included in version 4.2.0 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: