minor symfony#27355 Add code of Conduct links in our README (javiereguiluz)

This PR was submitted for the 2.7 branch but it was merged into the 2.8 branch instead (closes symfony#27355).

Discussion
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Add code of Conduct links in our README

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | no
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | -   <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | - <!-- required for new features -->

Commits
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40e59a6 Add code of Conduct links in our README

Author: nicolas-grekas

README.md

@@ -30,6 +30,7 @@ Community
 * [Join the Symfony Community][11] and meet other members at the [Symfony events][12].
 * [Get Symfony support][13] on Stack Overflow, Slack, IRC, etc.
 * Follow us on [GitHub][14], [Twitter][15] and [Facebook][16].
+* Read our [Code of Conduct][24] and meet the [CARE Team][25]
 
 Contributing
 ------------
@@ -72,3 +73,5 @@ Symfony development is sponsored by [SensioLabs][21], led by the
 [21]: https://sensiolabs.com
 [22]: https://symfony.com/doc/current/contributing/code/core_team.html
 [23]: https://github.com/symfony/symfony-demo
+[24]: https://symfony.com/coc
+[25]: https://symfony.com/doc/current/contributing/code_of_conduct/care_team.html