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>

Author: KevinOConnor

docs/CONTRIBUTING.md

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+# 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.

docs/developer-certificate-of-origin

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+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+1 Letterman Drive
+Suite D4700
+San Francisco, CA, 94129
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.