Add Markdown linting

Add linting for Markdown files, following norms from other repos.

Signed-off-by: Daniel Farrell <dfarrell@redhat.com>

.github/workflows/linting.yml

@@ -31,3 +31,15 @@ jobs:
         with:
           config-file: ".markdownlinkcheck.json"
           check-modified-files-only: "yes"
+
+  markdownlint:
+    name: Markdown
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out the repository
+        uses: actions/checkout@v2
+      - name: Run markdownlint
+        uses: nosborn/github-action-markdown-cli@v1.1.1
+        with:
+          files: .
+          config_file: ".markdownlint.yml"

.markdownlint.yml

@@ -0,0 +1,12 @@
+---
+# Breaks reusing MD snippets extracted to files
+first-line-heading: false
+
+# Set maximum line Length to 140c to match Go linting
+line-length:
+  line_length: 140
+
+# Allow HTML span elements to set font sizes
+no-inline-html:
+  allowed_elements:
+    - span