Add JetBrains sponsorship to README with H1 heading (#225)

Author: g105b

README.md

@@ -26,3 +26,9 @@ This repository is an implementation of the PSR-7 interfaces for use within PHP.
 </a>
 
 [fig]: https://www.php-fig.org/psr/psr-7/
+
+# 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/)