docs: Update contributing guidelines

Added developer certificate of origin.

Signed-off-by: Craig Bassett <craig.bassett@gmail.com>

CONTRIBUTING.md

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+# Contributing to Fluidd
+
+Fluidd exists as an independent client of Moonraker, and by extension - Klipper.
+Fluidd is built on VueJS, using TypeScript.
+
+- Source should always pass the linting rules defined, with no warnings or type errors.
+- A clean develop is preferred. This means squashing, and rebasing your feature branches prior to merge.
+- PR's should off a branch other than develop or master.
+- Commit messages should follow the [conventional commits](https://www.conventionalcommits.org/en/v1.0.0/) standard, and should have a Signed-off-by line, for example;
+
+  ```sh
+  feat: My feature.
+
+  Some description.
+
+  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).
+This must contain your real name and a current email address.

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.
+