github: add a CONTRIBUTING.md file that github shows when a PR or issue is filed

github links up CONTRIBUTING.md if it exists from the issue/PR filing form. Let's add one, in the hope that people have
a look before filing a PR.

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
new file mode 100644 (file)
index 0000000..ba4080c
--- /dev/null
+++ b/CONTRIBUTING.md
@@ -0,0 +1,38 @@
+# Contributing
+
+We welcome contributions from everyone. However, please follow the following guidelines when posting a GitHub Pull
+Request or filing a GitHub Issue on the systemd project:
+
+## Filing Issues
+
+* We use GitHub Issues **exclusively** for tracking **bugs** and **feature** **requests** of systemd. If you are
+  looking for help, please contact our [mailing list](http://lists.freedesktop.org/mailman/listinfo/systemd-devel)
+  instead.
+* We only track bugs in the **two** **most** **recently** **released** **versions** of systemd in the GitHub Issue
+  tracker. If you are using an older version of systemd, please contact your distribution's bug tracker instead.
+* When filing an issue, specify the **systemd** **version** you are experiencing the issue with. Also, indicate which
+  **distribution** you are using.
+* Please include an explanation how to reproduce the issue you are pointing out.
+
+Following these guidelines makes it easier for us to process your issue, and ensures we won't close your issue
+right-away for being misfiled.
+
+## Posting Pull Requests
+
+* Make sure to post PRs only relative to a very recent git master.
+* Follow our [Coding Style](https://raw.githubusercontent.com/systemd/systemd/master/CODING_STYLE) when contributing
+  code. This is a requirement for all code we merge.
+* Make sure to run "make check" locally, before posting your PR. We use a CI system, meaning we don't even look at your
+  PR, if the build and tests don't pass.
+
+* If you need to update the code in a existing PR, please consider opening a new PR (mentioning in it which old PR it
+  replaces) and closing the old PR. This is much preferable over force-pushing a new patch set into the PR's branch, as
+  commit comments aren't lost that way. That said, we don't follow this rule ourselves quite often, hence this is
+  really just a say as we say, not say as we do...
+
+## Final Words
+
+We'd like to apologize in advance if we are not able to process and reply to your issue or PR right-away. We have a lot
+of work to do, but we are trying our best!
+
+Thank you very much for your contributions!