Add changelog for 3.3.0-RC6

Author: Kordyjan

changelogs/3.3.0-RC6.md

@@ -0,0 +1,18 @@
+# Backported fixes
+
+- Patmat: Use less type variables in prefix inference [#16827](https//github.com/lampepfl/dotty/pull/16827)
+- Just warn on type ascription on a pattern [#17454](https://github.com/lampepfl/dotty/pull/17454)
+- Fix #17187: allow patches with same span [#17366](https://github.com/lampepfl/dotty/pull/17366)
+
+# Contributors
+
+Thank you to all the contributors who made this release possible 🎉
+
+According to `git shortlog -sn --no-merges 3.3.0-RC5..3.3.0-RC6` these are:
+
+```
+     2  Adrien Piquerez
+     2  Michał Pałka
+     2  Paweł Marks
+     1  Dale Wijnand
+```