Add an entry about i18n support

[alanpoulain]

See the articles:

• https://medium.com/locastic/having-troubles-with-implementing-translations-in-apiplatform-7861294fb7c6
• https://medium.com/@titouanbenoit/internationalization-with-api-platform-the-other-way-5ce9c446737f Related to:
• https://github.com/api-platform/core/issues/1506
• https://github.com/api-platform/core/issues/1694

[vasilvestre]

None of these ways is compliant with json-ld specs sadly and would require a big effort