docs: Update release procedure

Author: gberche-orange

CONTRIBUTING.adoc

@@ -6,6 +6,17 @@ tracker for issues and merging pull requests into master. If you want
 to contribute even something trivial please do not hesitate, but
 follow the guidelines below.
 
+== Releasing new version
+
+* Manually update `version='1.9.0'` in build.gradle files
+* Tag
+```bash
+git tag -f v1.9.0 -m "1.9.0 release"
+git push origin v1.9.0
+```
+
+Let github workflow produce artifacts associated to the release.
+
 === Code Conventions and Housekeeping
 None of these is essential for a pull request, but they will all help.  They can also be
 added after the original pull request but before a merge.