docs: Add contributing guidelines

Signed-off-by:  Eric Callahan <arksine.code@gmail.com>

docs/contributing.md

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+# Contributing to Moonraker
+
+While Moonraker exists as a service independently from Klipper, it relies
+on Klipper to be useful.  Thus, the tentative plan is to eventually merge
+the Moonraker application into the Klipper repo after Moonraker matures,
+at which point this repo will be archived.  As such, contibuting guidelines
+are near those of Klipper:
+
+- All source files should begin with a copyright notice in the following
+  format:
+  ```
+  # Module name and brief description of module
+  #
+  # Copyright (C) 2020 YOUR NAME <YOUR EMAIL ADDRESS>
+  #
+  # This file may be distributed under the terms of the GNU GPLv3 license
+  ```
+- No line in the source code or documentation should exceed 80 characters.
+  Be sure there is no trailing whitespace.
+- Each Commit message should be in the following format:
+  ```
+  module name: brief description of commit
+
+  More detailed explanation if necessary.
+
+  Signed-off-by:  Your Name <your email address>
+  ```
+- By signing off on commits, you acknowledge that you agree to the
+  [developer certificate of origin](developer-certificate-of-origin).
+  As with Klipper, this must contain your real name and 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.