Add ci-cla-check

.github/workflows/ci-cla-check.yaml

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+name: cla-check
+on:
+  pull_request:
+    branches: [ "master" ]
+
+jobs:
+  cla-check:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check if CLA signed
+        uses: canonical/has-signed-canonical-cla@v2

CONTRIBUTING.md

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+# Contributing to secboot
+
+We are an open source project and welcome community contributions, suggestions,
+fixes and constructive feedback.
+
+If you'd like to contribute, you will first need to sign the Canonical
+contributor agreement. This is the easiest way for you to give us permission to
+use your contributions. In effect, you’re giving us a licence, but you still
+own the copyright — so you retain the right to modify your code and use it in
+other projects.
+
+The agreement can be found, and signed, here:
+https://ubuntu.com/legal/contributors
+
+## Contributor guidelines
+
+Contributors can help us by observing the following guidelines:
+
+- Commit messages should be well structured.
+- Commit emails should not include non-ASCII characters.
+- Several smaller PRs are better than one large PR.
+- Try not to mix potentially controversial and trivial changes together.
+  (Proposing trivial changes separately makes landing them easier and 
+  makes reviewing controversial changes simpler)
+- Do not force push a PR after it has received reviews. It is
+  acceptable to force push when a PR is ready to merge, however.
+- Try to write tests to cover the contributed changes (see below)
+
+## Pull request guidelines
+
+Contributions are submitted through a pull request created from
+a fork of the `secboot` repository (under your GitHub account).

PULL_REQUEST_TEMPLATE.md

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+Thanks for helping us make a better secboot!
+Have you signed the [license agreement](https://www.ubuntu.com/legal/contributors) and read the [contribution guide](CONTRIBUTING.md)?