minor symfony#41286 Add sponsors for 5.4 (fabpot)

This PR was merged into the 5.4 branch.

Discussion
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Add sponsors for 5.4

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | n/a
| License       | MIT
| Doc PR        | n/a

/cc `@Seldaek` `@naderman`

Commits
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f0c3389 Add sponsors for 5.4

Author: fabpot

README.md

@@ -14,6 +14,17 @@ Installation
   Support" (LTS) versions and has a [release process][6] that is predictable and
   business-friendly.
 
+Sponsor
+-------
+
+Symfony 5.4 is [backed][27] by [Private Packagist][28].
+
+Private Packagist is a fast, reliable, and secure Composer repository for your
+private packages. It mirrors all your open-source dependencies for better
+availability and monitors them for security vulnerabilities.
+
+Help Symfony by [sponsoring][29] its development!
+
 Documentation
 -------------
 
@@ -75,3 +86,6 @@ Symfony development is sponsored by [SensioLabs][21], led by the
 [24]: https://symfony.com/coc
 [25]: https://symfony.com/doc/current/contributing/code_of_conduct/care_team.html
 [26]: https://symfony.com/book
+[27]: https://symfony.com/backers
+[28]: https://packagist.com/
+[29]: https://symfony.com/sponsor