Add CONTRIBUTING.md

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

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+# LTTng-UST contributor's guide
+
+Being an open source project, the LTTng-UST project welcomes
+contributions from anyone. This guide walks you through the process
+of contributing a patch to LTTng-UST.
+
+
+## Getting the source code
+
+The LTTng-UST project uses [Git](https://git-scm.com/) for version
+control. The upstream Git repository URL is:
+
+    git://git.lttng.org/lttng-ust.git
+
+
+## Coding standard
+
+LTTng-UST uses the
+[Linux kernel coding style](http://www.kernel.org/doc/Documentation/CodingStyle).
+
+Although the LTTng-UST code base is primarily written in C, it does
+contain shell, Perl, and Python code as well. There is no official coding
+standard for these languages. However, using a style consistent with the
+rest of the code written in that language is strongly encouraged.
+
+
+## Creating and sending a patch
+
+LTTng-UST's development flow is primarily email-based, although we
+also accept pull requests on our
+[GitHub mirror](https://github.com/lttng/lttng-ust). If you're going
+to create GitHub pull requests, make sure you still follow the
+guidelines below.
+
+Like a lot of open source projects, patches are submitted and reviewed
+on its development mailing list,
+[`lttng-dev`](http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev)
+(`lttng-dev@lists.lttng.org`). The mailing list is also used to share
+and comment on <abbr title="Request for Comments">RFC</abbr>s and answer
+user questions.
+
+Once your changes have been committed to your local branch, you may use
+Git's [`format-patch`](https://git-scm.com/docs/git-format-patch) command
+to generate a patch file. The following command line generates a
+patch from the latest commit:
+
+    git format-patch -N1 -s --subject-prefix="PATCH lttng-ust"
+
+The custom `PATCH lttng-ust` subject prefix is mandatory when
+submitting patches that apply to the LTTng-UST project.
+
+The patch's subject (the commit message's first line) should:
+
+  * begin with an uppercase letter
+  * be written in the present tense
+  * _not_ exceed 72 characters in length
+  * _not_ end with a period
+  * be prefixed with `Fix:` if the commit fixes a bug
+
+The commit message's body should be as detailed as possible and explain
+the reasons behind the proposed change. Any related
+[bug report(s)](https://bugs.lttng.org/projects/lttng-ust/issues)
+should be mentioned at the end of the message using the `#123` format,
+where `123` is the bug number:
+
+  * Use `Refs: #123` if the patch is related to bug 123, but does not
+    fix it yet.
+  * Use `Fixes: #123` to signify that this patch fixes the bug.
+
+Make sure to **sign-off** your submitted patches (the `-s` argument to
+Git's `commit` and `format-patch` commands).
+
+Here's a complete example:
+
+~~~ text
+Fix: use this instead of that in some context
+
+Ball tip jowl beef ribs shankle, leberkas venison turducken tail pork
+chop t-bone meatball tri-tip. Tongue beef ribs corned beef ball tip
+kevin ground round sausage rump meatloaf pig meatball prosciutto
+landjaeger strip steak. Pork pork belly beef.
+
+Biltong turkey porchetta filet mignon corned beef. T-bone bresaola
+shoulder meatloaf tongue kielbasa.
+
+Fixes: #321
+Refs: #456
+Refs: #1987
+
+Signed-off-by: Jeanne Mance <jmeance@lttng.org>
+~~~
+
+Please note that patches should be **as focused as possible**. Do not,
+for instance, fix a bug and correct the indentation of an unrelated
+block of code as part of the same patch.
+
+Once you are confident your patch meets the required guidelines,
+you may use Git's [`send-email`](https://git-scm.com/docs/git-send-email)
+command to send your patch to the mailing list:
+
+    git send-email --suppress-cc=self --to lttng-dev@lists.lttng.org *.patch
+
+Make sure you are
+[subscribed](http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev)
+to the mailing list to follow and take part in discussions about your
+changes. You may join the file to an email as an attachment if you can't
+send the patch directly using <code>git&nbsp;send&#8209;email</code>.
+
+
+## Reviews
+
+Once your patch has been posted to the mailing list or as a GitHub
+pull request, other contributors may propose modifications.
+This is completely normal. This collaborative code review is an integral
+part of the open source development process in general and LTTng-UST
+makes no exception.
+
+Keep in mind that reviewing patches is a time-consuming process and,
+as such, may not be done right away. The delays may be affected by the
+current release cycle phase and the complexity of the proposed changes.
+If you think your patch might have been forgotten, please mention it on
+the [`#lttng`](irc://irc.oftc.net/lttng) IRC channel rather than
+resubmitting.
+
+
+## Release cycle
+
+The LTTng-UST project follows a release cycle that alternates between
+development and release candidate (RC) phases. The master branch is
+feature-frozen during RC phases: only bug fixes are accepted during
+this period. However, patches adding new functionality may still be
+submitted and reviewed during the RC. The upcoming features and release
+dates are posted in a monthly digest on the mailing list.