add CONTIBUTING.md

add a little description about how to contribute to the Xserver.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

CONTIBUTING.md

@@ -0,0 +1,39 @@
+CONTRIBUTING TO XSERVER
+======================
+
+Preparation
+-----------
+
+* become confident with git and gitlab (if you aren't already)
+* join the xorg-devel mailing list: https://lists.freedesktop.org/mailman/listinfo/xorg
+* create an gitlab account on https://gitlab.freedesktop.org
+* request fork permissions: https://gitlab.freedesktop.org/freedesktop/freedesktop/-/wikis/home#how-can-i-contribute-to-an-existing-project-or-create-a-new-one
+* fork xserver repo: https://gitlab.freedesktop.org/xorg/xserver
+
+Patch/commit quality
+--------------------
+
+* each commit should solve one particular problem and be self-consistent, ie. complete in itself and not break anything else
+* preparational steps should be done in separate commits
+* describe what the change does and why it's necessary within the commit message
+* commit title should contain prefix telling which parts are touched (eg. "modesetting: ", "xwayland: ", "Xext: ", ...)
+* if a commit fixes an issue (ticket), add an "Closes: ..." header (with link to issue) to the commit message
+* if a commit fixes an regression by another commit, add an "Fixes: " header (with broken commit's id) to commit message
+* commits to be merged need to by signed-off by the author
+* example: https://gitlab.freedesktop.org/xorg/xserver/-/commit/3ddb81b
+
+Submission
+----------
+
+* separate issues should be submitted separately (as separate branches)
+* file a pull request in gitlab
+* if it should be backported to other branches (eg. 21.x releases), mention this is the PR description
+* use the "draft" flag if the submission is just for review/discussion instead of merging to mainline
+* make sure each individual commits builds all tests are green (important for bisect)
+* be patient !
+
+Further reading
+---------------
+
+* https://www.x.org/wiki/Development/Documentation/SubmittingPatches/
+* https://gitlab.freedesktop.org/freedesktop/freedesktop/-/wikis/home#how-can-i-contribute-to-an-existing-project-or-create-a-new-one