CONTRIBUTING: Add to which branches to submit PRs

CONTRIBUTING.md

@@ -2,8 +2,31 @@
 
 The SPDX specification is maintained by the SPDX [legal][spdx-legal] and [tech][spdx-tech] teams.
 Design and planning is primarily done via the team [mailing][spdx-legal-list] [lists][spdx-tech-list] and meetings.
-You may want to discuss significant changes on the mailing list first to get design feedback before investing time in a pull request.
+
+## Submitting changes
+
+Always write a clear log message for your commits. One-line messages are fine for small changes, but significant changes should look like this:
+
+    $ git commit -m "Subject of the commit
+    > 
+    > A paragraph describing what changed and its impact."
+
+A properly formed Git commit subject line should always be able to complete the following sentence: if applied, this commit will "Subject of the commit". For example :
+
+    if applied, this commit will Add chapter on Security Vunerabilities in SPDX
+    if applied, this commit will Delete section with deprecated SPDX attributes 
+    if applied, this commit will Fix grammar in SPDX 3.3 Package Version
+
+Git itself use this approach. When you merge something it will generate a commit message like "Merge branch...", or when reverting "Revert...".
+
+### Minor Changes
 Minor changes such as markup and typo fixes may be submitted directly to this repository (either as [issues][] or [pull-requests][]) without previous discussion.
+Please submit all minor changes against the `development/v2.2` branch which is the draft of the next version of the SPDX specification to be released.
+
+### Major Changes
+Any change that break backwards compatibility or requires significant tooling changes is considered a major change.
+You may want to discuss major changes on the mailing list first to get design feedback before investing time in a pull request.
+Please submit all major changes against the `development/v3.0` which is the next major version of the specification. 
 
 [issues]: https://github.com/spdx/spdx-spec/issues/
 [pull-requests]: https://github.com/spdx/spdx-spec/pulls/