Update changelog for 3.12.0 release

Author: acrobat

CHANGELOG-3.X.md

@@ -9,6 +9,15 @@
 ### Changed
 - Fixed branch alias ([GrahamCampbell](https://github.com/GrahamCampbell)) [#1109](https://github.com/KnpLabs/php-github-api/issues/1109)
 
+## 3.12.0
+
+### Added
+- feat: Support for Organization Runners ([haridarshan](https://github.com/haridarshan), [renovate](https://github.com/renovate)[[bot](https://github.com/bot)]) [#1101](https://github.com/KnpLabs/php-github-api/issues/1101)
+- allow psr/http-message v2 ([LordSimal](https://github.com/LordSimal)) [#1122](https://github.com/KnpLabs/php-github-api/issues/1122)
+
+### Changed
+- Fixed branch alias ([GrahamCampbell](https://github.com/GrahamCampbell)) [#1109](https://github.com/KnpLabs/php-github-api/issues/1109)
+
 ## 3.11.0
 
 ### Added