github: lint markdown without enforcement

Signed-off-by: Philip Molloy <philip@philipmolloy.com>

.github/workflows/markdownlint.yml

@@ -0,0 +1,12 @@
+name: Markdown Lint Action
+on: push
+
+jobs:
+  build:
+    name: Markdown Lint
+    runs-on: ubuntu-latest
+    steps:
+      - name: Install markdown lint
+        run: sudo gem install mdl
+      - name: Lint markdown
+        run: mdl $(find . -name *.md)