symfony

This PR was merged into the 4.4 branch.

Discussion
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Update README.md

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no<!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | (see below)
| License       | MIT
| Doc PR        | /

Replace `composer install` by `composer require` because composer uses the command `require` to add a dependancy
**cf :** https://github.com/symfony/debug/pull/9

symfony/debug does not exist for symfony 5 or above ?

Commits
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24d372a9 Update README.md