Add DCO/signoff docs and sign off on MAINTAINERS entry

Signed-off-by: Brian S. Stephan <bss@incorporeal.org>

CONTRIBUTING.md

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+# Contributing Guidelines
+
+gp2040ce-binary-tools is made available under the MIT (Expat) license. Contributions are welcome via pull requests. This
+document outlines the process to get your contribution accepted.
+
+## Sign Offs/Custody of Contributions
+
+The Developer Certificate of Origin (DCO) is a way for contributors to certify that they wrote or otherwise have the
+right to license their code contributions to the project. The full text of the DCO can be found
+[here](https://developercertificate.org/) or in `DCO.txt`. Contributors must sign-off that they adhere to these
+requirements by adding a `Signed-off-by` line to their commit message, and/or, for frequent contributors, by signing off
+on their entry in `MAINTAINERS.md`.
+
+This process is followed by a number of open source projects, most notably the Linux kernel. Here's the gist of it:
+
+```
+[Your normal Git commit message here.]
+
+Signed-off-by: Random J Developer <random@developer.example.org>
+```
+
+`git help commit` has more info on adding this:
+
+```
+-s, --signoff
+    Add Signed-off-by line by the committer at the end of the commit log
+    message. The meaning of a signoff depends on the project, but it typically
+    certifies that committer has the rights to submit this work under the same
+    license and agrees to a Developer Certificate of Origin (see
+    http://developercertificate.org/ for more information).
+```

DCO.txt

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+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+
+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.

MAINTAINERS.md

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+# Maintainers
+
+This file contains information about people permitted to make major decisions and direction on the project.
+
+## Contributing Under the DCO
+
+By adding your name and email address to this section, you certify that all of your subsequent contributions to
+gp2040ce-binary-tools are made under the terms of the Developer's Certificate of Origin 1.1, available at `DCO.txt`.
+
+* Brian S. Stephan (<bss@incorporeal.org>)