ci: create an initial GitHub actions workflow (#390)

This workflow does nothing, but we need an initial workflow in place in
order to be able to iterate and test with changes to that workflow in
later PRs. Hence we commit a simple "Hello, World!" workflow for now
that will be refined in later PRs.

.github/workflows/test.yml

@@ -0,0 +1,18 @@
+name: CI
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - '**'
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@722adc63f1aa60a57ec37892e133b1d319cae598
+    - name: Run a one-line script
+      run: echo Hello, world!