Add CI workflow to lint YAML files

On every push and pull request that affects relevant files, and periodically, run yamllint to check the YAML files of
the repository for issues.

The .yamllint.yml file is used to configure yamllint:
https://yamllint.readthedocs.io/en/stable/configuration.html

.github/workflows/check-yaml.yml

@@ -0,0 +1,78 @@
+name: Check YAML
+
+# See: https://docs.github.com/en/free-pro-team@latest/actions/reference/events-that-trigger-workflows
+on:
+  push:
+    paths:
+      - ".github/workflows/check-yaml.yml"
+      - ".yamllint*"
+      # Source: https://github.com/ikatyang/linguist-languages/blob/master/data/YAML.json (used by Prettier)
+      - "**/.clang-format"
+      - "**/.clang-tidy"
+      - "**/.gemrc"
+      - "**/glide.lock"
+      - "**.yml"
+      - "**.mir"
+      - "**.reek"
+      - "**.rviz"
+      - "**.sublime-syntax"
+      - "**.syntax"
+      - "**.yaml"
+      - "**.yaml-tmlanguage"
+      - "**.yaml.sed"
+      - "**.yml.mysql"
+  pull_request:
+    paths:
+      - ".github/workflows/check-yaml.yml"
+      - ".yamllint*"
+      - "**/.clang-format"
+      - "**/.clang-tidy"
+      - "**/.gemrc"
+      - "**/glide.lock"
+      - "**.yml"
+      - "**.mir"
+      - "**.reek"
+      - "**.rviz"
+      - "**.sublime-syntax"
+      - "**.syntax"
+      - "**.yaml"
+      - "**.yaml-tmlanguage"
+      - "**.yaml.sed"
+      - "**.yml.mysql"
+  schedule:
+    # Run every Tuesday at 8 AM UTC to catch breakage caused by changes to yamllint.
+    - cron: "0 8 * * TUE"
+  workflow_dispatch:
+  repository_dispatch:
+
+jobs:
+  check:
+    name: ${{ matrix.configuration.name }}
+    runs-on: ubuntu-latest
+
+    strategy:
+      fail-fast: false
+
+      matrix:
+        configuration:
+          - name: Generate problem matcher output
+            # yamllint's "github" output type produces annotated diffs, but is not useful to humans reading the log.
+            format: github
+            # The other matrix job is used to set the result, so this job is configured to always pass.
+            continue-on-error: true
+          - name: Check formatting
+            # yamllint's "colored" output type is most suitable for humans reading the log.
+            format: colored
+            continue-on-error: false
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Check YAML
+        continue-on-error: ${{ matrix.configuration.continue-on-error }}
+        run: |
+          yamllint \
+            --config-file "${{ github.workspace }}/.github/.yamllint.yml" \
+            --format ${{ matrix.configuration.format }} \
+            .