examples: add invariant validation check

Author: inaciovasquez2020

scripts/check_examples.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+FILES=$(find examples -type f -name "*.md" ! -name README.md)
+
+for f in $FILES; do
+  grep -q "Example:" "$f"
+  grep -q "Invariant" "$f"
+done