doc: add downstream CONTRIBUTING document

rhel-only

Related: #2179309

CONTRIBUTING.md

@@ -0,0 +1,67 @@
+# Contributing
+
+Welcome to systemd source-git for CentOS Stream and RHEL. When contributing, please follow the guide below.
+
+## Workflow
+
+```mermaid
+flowchart LR
+    A(Issue) --> B{is fixed\nupstream}
+    B -->|YES| C(backport\nupstream patch)
+    B -->|NO| D(upstream\nsubmit issue or PR)
+    D --> E{accepted\nand fixed}
+    E -->|YES| C
+    E -->|NO| F(rhel-only patch) --> G
+    C --> G(submit PR)
+```
+
+## Filing issues
+
+When you find an issue with systemd used in CentOS Stream or RHEL, please file an issue in [Jira ticket system](https://issues.redhat.com/secure/CreateIssue!default.jspa) (set Project to **RHEL** and Component to **systemd**).
+
+GitHub Issues are not supported tracking system. If your issue is reproducible using the latest upstream version of systemd, please consider creating [upstream issue](https://github.com/systemd/systemd/issues/new/choose).
+
+## Posting Pull Requests
+
+Every Pull Request has to comply with the following rules:
+
+- Each commit has to reference [upstream](https://github.com/systemd/systemd) commit.
+- Each commit has to reference the approved issue/tracker.
+- Pull requests have to pass mandatory CI validation and testing
+- Pull requests have to be approved by at least one systemd downstream maintainer
+
+### Upstream reference
+
+When doing a back-port of an upstream commit, always use `cherry-pick -x <sha>`. Consider proposing a change upstream first when an upstream commit doesn't exist.
+If the change isn't upstream relevant or accepted by upstream, mark the commit with the `rhel-only` string.
+
+```md
+doc: Fix TYPO
+
+rhel-only
+
+Resolves: RHEL-678
+```
+
+### Issue reference
+
+Each commit has to reference the relevant approved systemd issue (see: [Filling issues section](#filing-issues)). For referencing issues, we use the following keywords:
+
+- **Resolves** for commits that directly resolve issues described in a referenced tracker
+- **Related** for commits related to the referenced issue, but they don't fix it. Usually, tests and documentation.
+- **Reverts** for commits that reverts previously merged commit
+
+When referencing issues, use following structure: `<keyword>: <issue ID>`. See the example below:
+
+```md
+doc: Fix TYPO
+
+(cherry picked from commit c5afbac31bb33e7b1f4d59b253425af991a630a4)
+
+Resolves: RHEL-678
+```
+
+### Validation and testing
+
+Each Pull Request has to pass all enabled tests that are automatically run using GitHub Actions, CentOS Stream CI, and others.
+If CI failure is unrelated to the change introduced in Pull Request, the downstream maintainer will set the `ci-waived` label and explain why CI was waived.