docs: Add a CONTRIBUTING document

Add an initial document detailing best practices for contributing to
the document.  Add the "developer certificate of origin" document.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/CONTRIBUTING.md

@@ -0,0 +1,38 @@
+# Contributing to Klipper
+
+Thank you for contributing to Klipper! Please take a moment to read
+this document.
+
+## Creating a new issue
+
+Please see the [contact page](Contact.md) for information on creating
+an issue. In particular, **we need the klippy.log file** attached to
+bug reports. Also, be sure to read the [FAQ](FAQ.md) to see if a
+similar issue has already been raised.
+
+## Submitting a pull request
+
+Contributions of Code and documentation are managed through github
+pull requests.  Each commit should have a commit message formatted
+similar to the following:
+
+```
+module: Capitalized, short (50 chars or less) summary
+
+More detailed explanatory text, if necessary.  Wrap it to about 75
+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.
+
+Further paragraphs come after blank lines..
+
+Signed-off-by: My Name <myemail@example.org>
+```
+
+It is important to have a "Signed-off-by" line on each commit - it
+certifies that you agree to the
+[developer certificate of origin](developer-certificate-of-origin). It
+must contain your real name (sorry, no pseudonyms or anonymous
+contributions) and contain a current email address.