doc: add README files for components (#6653)

* docs: add README for components

* Update README.md

Co-authored-by: Antoine Bluchet <[email protected]>

* Update README.md

Co-authored-by: Antoine Bluchet <[email protected]>

* Update README.md

Co-authored-by: Antoine Bluchet <[email protected]>

* remove trailing spaces

* typo

* better title

---------

Co-authored-by: Antoine Bluchet <[email protected]>

Author: dunglas

README.md

@@ -0,0 +1,14 @@
+# API Platform - JSON:API
+
+The [JSON:API](https://jsonapi.org/) component of the [API Platform](https://api-platform.com) framework.
+
+JSON:API is a popular specification for building APIs in JSON.
+
+[Documentation](https://api-platform.com/docs/core/content-negotiation/)
+
+> [!CAUTION]
+>
+> This is a read-only sub split of `api-platform/core`, please
+> [report issues](https://github.com/api-platform/core/issues) and
+> [send Pull Requests](https://github.com/api-platform/core/pulls)
+> in the [core API Platform repository](https://github.com/api-platform/core).