Add workflow to welcome first-time contributors (#50)

This adds a workflow that triggers on new issues and PRs. If the author
has not contributed to this repository before, the workflow will add a
comment with an informative welcome message to the issue or PR. The goal
is to try to be welcoming of contributions and simultaneously steer
people towards information they may have missed.

.github/workflows/first-interaction.yaml

@@ -0,0 +1,85 @@
+# Summary: add a welcoming comment to first-time contributors' issues & PRs.
+# This is written in a generic way so that we can use the same workflow
+# in all our quantumlib repos.
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+name: Welcome first interactions
+run-name: Welcome ${{github.actor}}'s first interaction
+
+on:
+  issues:
+    types: [opened]
+  pull_request:
+    types: [opened]
+
+# Declare default permissions as read-only.
+permissions: read-all
+
+jobs:
+  welcome:
+    name: Check for first interaction
+    runs-on: ubuntu-24.04
+    timeout-minutes: 10
+    permissions:
+      issues: write
+      pull-requests: write
+    env:
+      repo: ${{github.server_url}}/${{github.repository}}
+      files: ${{github.server_url}}/${{github.repository}}/blob/${{github.ref_name}}
+    steps:
+      - name: Check out a copy of the git repository
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+
+      - name: Add a welcome comment if appropriate
+        uses: actions/first-interaction@2ec0f0fd78838633cd1c1342e4536d49ef72be54 # v1.3.0
+        with:
+          repo-token: ${{secrets.GITHUB_TOKEN}}
+          issue-message: |
+            Hello and welcome, ${{github.actor}} 👋!
+
+            Thanks for filing your first issue with the ${{github.repository}}
+            project! We are glad you are here and are excited to see your first
+            contribution. Please keep in mind our [community participation
+            guidelines](${{env.files}}/CODE_OF_CONDUCT.md).
+
+            If this is a bug report, we will probably need the following
+            details in order to diagnose the issue. If you did not include it
+            in the original issue description, please consider providing it
+            now because it will probably save everyone time in the long run:
+
+            - [ ] Environment you are using (MacOS, Windows, Colab, etc.)
+            - [ ] Version of Python you are using
+            - [ ] Steps to reproduce the issue
+
+            Please allow time for the project community to be able to read the
+            issue and act on it. In the meantime, you may want to look through
+            the [existing open issues](${{env.repo}}/issues) to see if any look
+            similar to yours. If you find a similar or identical issue,
+            consider closing this one; if you don't want to close this one, it
+            would be helpful if you could write a comment here that references
+            the related issue and explains how this new one differs from the
+            other one.
+
+            This is an automatically-generated message.
+          pr-message: |
+            Hello and welcome, ${{github.actor}} 👋!
+
+            Thanks for making your first pull request to the
+            ${{github.repository}} project! We are glad you are here and are
+            excited to see your first contribution. Please keep in mind our
+            [community code of conduct](${{env.files}}/CODE_OF_CONDUCT.md).
+
+            Since this is your first pull request with this project, the
+            automated check for the Google [Contributor License
+            Agreement](https://cla.developers.google.com/about) (CLA) will
+            trigger and may request that you to sign the CLA. More information
+            about the agreement can be found in the project's [contributing
+            guide](${{env.files}}/CONTRIBUTING.md).
+
+            If this pull request is to fix a bug, please reference the relevant
+            issue number in the project [issue tracker](${{env.repo}}/issues).
+
+            Please allow time for the project community to be able to read and
+            evaluate your pull request.
+
+            This is an automatically-generated message.