Add CONTRIBUTING.md file


Point potential contributors to contribute.qt-project.org.

Change-Id: I975cd9537f1f8a5e337cf1af7dea1704241fcf36
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
commitd4abd9a515b9c47ff538ea1cf69f6aec98c363d9
parente4b51b8e74a9e4e4215fffcdcf82608c4d297261
authorKai Köhne <kai.koehne@qt.io>
committerKai Köhne <kai.koehne@qt.io>
commit timeWed, 22 Apr 2026 15:48:52 +0000