chore: Add a DCO check. (#987)

.github/workflows/check-dco.yaml

@@ -0,0 +1,21 @@
+name: Check DCO
+
+on:
+  pull_request:
+  merge_group:
+
+jobs:
+  check-dco:
+    permissions:
+      checks: write
+      contents: read
+      pull-requests: read
+      statuses: write
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check DCO
+        uses: hackworthltd/[email protected]
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}

DCO.md

@@ -0,0 +1,34 @@
+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.

README.md

@@ -15,6 +15,35 @@ Note that Primer must be built with GHC 9.2.1 or later.
 We currently support two SQL backend implementations: SQLite and
 PostgreSQL.
 
+# License & contributing
+
+This implementation of the Primer programming language is licensed
+under [version 3 of the GNU Affero Public License](COPYING). We
+welcome contributions from the open source community, so long as you
+agree to abide by our [code of conduct](CODE_OF_CONDUCT.md), and
+certify that your contribution is made according to the terms of
+version 1.1 of The Linux Foundation's [Developer Certificate of
+Origin](DCO.md).
+
+As is customary for open source projects that adhere to the DCO, we
+ask that you use the `--signoff` flag of `git commit` when submitting
+contributions, in order to signify your agreement. Should you forget
+to use this flag when submitting a contribution, in most cases,
+retroactively applying it is relatively easy; for any given series of
+commits in a branch, you can usually run:
+
+```sh
+git rebase HEAD~N --signoff
+```
+
+where `N` is the number of individual commits in your branch.
+
+For background on why we request that contributions are made under the
+DCO, please see the following links:
+
+* https://wiki.openstack.org/wiki/OpenStackAndItsCLA
+* https://lwn.net/Articles/592503/
+
 # Developing Primer
 
 ## Nix