Add CI workflow to check for missed updates to readme ToC

On every push or pull request that affects the repository's README.md, check whether the table of contents matches the
content.

Author: per1234

.github/workflows/check-toc.yml

@@ -0,0 +1,46 @@
+name: Check ToC
+
+# See: https://docs.github.com/en/actions/reference/events-that-trigger-workflows
+on:
+  push:
+    paths:
+      - ".github/workflows/check-toc.ya?ml"
+      - "README.md"
+  pull_request:
+    paths:
+      - ".github/workflows/check-toc.ya?ml"
+      - "README.md"
+  workflow_dispatch:
+  repository_dispatch:
+
+jobs:
+  check:
+    name: ${{ matrix.file.name }}
+    runs-on: ubuntu-latest
+
+    strategy:
+      fail-fast: false
+
+      matrix:
+        file:
+          - name: README.md
+            # Max ToC depth, for use with the markdown-toc --maxdepth flag.
+            maxdepth: 3
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Install markdown-toc
+        run: sudo npm install --global markdown-toc
+
+      - name: Rebuild ToC
+        run: |
+          markdown-toc \
+            --bullets=- \
+            --maxdepth=${{ matrix.file.maxdepth }} \
+            -i \
+            "${{ github.workspace }}/${{ matrix.file.name }}"
+
+      - name: Check ToC
+        run: git diff --color --exit-code