[enh] #423: Add mdformat pre-commit hook

.pre-commit-config.yaml

@@ -39,3 +39,7 @@ repos:
         args:
         - --license-filepath
         - license_header.txt
+-   repo: https://github.com/executablebooks/mdformat
+    rev: 0.7.14
+    hooks:
+      - id: mdformat