Add JetBrains sponsorship to README with H1 heading

Author: g105b

README.md

@@ -69,3 +69,9 @@ Working with JSON data
 A JSON data structure is almost identical in scope to the DataObject introduced in this repository, with one key difference: JSON data can represent a primitive data type, not always key-value-pairs. Because of this, [PHP.Gt/Json is maintained separately to provide structured, type-safe, immutable JSON objects][json] as an extension to this DataObject repository.
 
 [json]: https://php.gt/json
+
+# Proudly sponsored by
+
+[JetBrains Open Source sponsorship program](https://www.jetbrains.com/community/opensource/)
+
+[![JetBrains logo.](https://resources.jetbrains.com/storage/products/company/brand/logos/jetbrains.svg)](https://www.jetbrains.com/community/opensource/)