.github: add contributing guidelines

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

.github/CONTRIBUTING.md

@@ -0,0 +1,90 @@
+Contributing to the Project
+===========================
+
+Thank you for considering contributing back to [Free Software][1]!
+
+There are a few things we would like you to consider when filing an
+issue or pull request with this project:
+
+1. If you are filing a bug report or feature request
+
+   Please take the time to check if an issue already has been filed
+   matching your problem
+
+2. What version are you running, have you tried the latest release?
+
+   UNIX distributions often package and test software for their
+   particular brand.  If you are using a pre-packaged version,
+   then please file a bug with that distribution instead.
+
+3. Coding Style
+
+   Lines are allowed to be longer than 72 characters these days, there
+   is no enforced max. length.
+
+> **Tip:** Always submit code that follows the style of surrounding code!
+
+   The coding style itself is strictly Linux [KNF][], like GIT it is
+   becoming a de facto standard for C programming
+
+   https://www.kernel.org/doc/Documentation/CodingStyle
+
+4. Logical Change Sets
+
+   Changes should be broken down into logical units that add a feature
+   or fix a bug.  Keep changes separate from each other and do not mix a
+   bug fix with a whitespace cleanup or a new feature addition.
+
+   This is important not only for readability, or for the possibility of
+   maintainers to revert changes, but does also increase your chances of
+   having a change accepted.
+
+5. Commit messages
+
+   Commit messages exist to track *why* a change was made.  Try to be as
+   clear and concise as possible in your commit messages, and always, be
+   proud of your work and set up a proper GIT identity for your commits:
+
+        git config --global user.name "J. Random Hacker"
+        git config --global user.email random.j.hacker@example.com
+
+   See this helpful guide for how to write simple, readable commit
+   messages, or have at least a look at the below example.
+
+   http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
+
+
+Example
+-------
+
+Example commit message from the [Pro Git][gitbook] online book, notice
+how `git commit -s` is used to automatically add a `Signed-off-by`:
+
+    Capitalized, short (50 chars or less) summary
+
+    More detailed explanatory text, if necessary.  Wrap it to about 72
+    characters or so.  In some contexts, the first line is treated as the
+    subject of an email and the rest of the text as the body.  The blank
+    line separating the summary from the body is critical (unless you omit
+    the body entirely); tools like rebase can get confused if you run the
+    two together.
+
+    Write your commit message in the imperative: "Fix bug" and not "Fixed bug"
+    or "Fixes bug."  This convention matches up with commit messages generated
+    by commands like git merge and git revert.
+
+    Further paragraphs come after blank lines.
+
+    - Bullet points are okay, too
+
+    - Typically a hyphen or asterisk is used for the bullet, followed by a
+      single space, with blank lines in between, but conventions vary here
+
+    - Use a hanging indent
+
+    Signed-off-by: J. Random Hacker <random.j.hacker@example.com>
+
+
+[1]:       http://www.gnu.org/philosophy/free-sw.en.html
+[KNF]:     https://en.wikipedia.org/wiki/Kernel_Normal_Form
+[gitbook]: https://git-scm.com/book/ch5-2.html