Add GitHub Action for checking links in Markdown content.

.github/dictionary.txt

@@ -0,0 +1,3 @@
+CommonMark
+markdownlint-cli2-action
+README

.github/workflows/checkers.yml

@@ -0,0 +1,27 @@
+name: Checkers
+
+on:
+  pull_request:
+  push:
+  schedule:
+  - cron: '30 12 * * *'
+  workflow_dispatch:
+
+jobs:
+
+  linkcheck:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - uses: JustinBeckwith/linkinator-action@v1.10.2
+      with:
+        paths: '*.md'
+
+  spellcheck:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - uses: tbroadley/spellchecker-cli-action@v1.1.1
+      with:
+        dictionaries: '.github/dictionary.txt'
+        files: '*.md'