Update contributing

AGENTS.md

@@ -325,7 +325,11 @@ docstrings, log and console text, commit messages, issue text, and replies.
   comment style of the file: `Copyright <year> 514 LLC d/b/a OpenGlow` and
   `Written by Scott Wiederhold`. Keep the year, or the year range, that
   the file already has. The copyright notices of other holders stay
-  unchanged.
+  unchanged. Those two lines are for files this project authors: an
+  outside contributor who writes a new file puts their own name there, or
+  leaves the lines out, and keeps their copyright in what they wrote.
+  Editing an existing file changes no header - a contribution is recorded
+  by its `Signed-off-by` line and by git, not by a copyright line.
 - **SPDX.** Every new source file carries an `SPDX-License-Identifier` line
   under the license of its repository.
 

CONTRIBUTING.md

@@ -0,0 +1,54 @@
+# Contributing
+
+Patches are welcome. How the project works, what proof a change needs, and the
+writing rules are on the documentation site:
+<https://docs.forgefirm.org/developers/contributing/>.
+
+## Sign your work
+
+Every commit carries a `Signed-off-by` line:
+
+    Signed-off-by: Your Name <your.email@example.com>
+
+`git commit -s` adds it, using your `user.name` and `user.email`. Use your real
+name; an anonymous or pseudonymous sign-off certifies nothing.
+
+The line certifies the Developer Certificate of Origin, version 1.1, below. It
+is the same statement the Linux kernel and grblHAL use. It is not a copyright
+assignment: you keep the copyright in what you write, and your contribution
+comes in under the license of the repository you are contributing to.
+
+You do not add a copyright line for a patch. A change to an existing file
+leaves its header alone. If you write a whole new file, put your own name in
+its copyright line, or leave the line out; either way the file carries an
+`SPDX-License-Identifier` for the repository's license.
+
+## Developer 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.
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+This DCO text is licensed under the terms of the Creative Commons
+Attribution-ShareAlike 3.0 Unported license.