Add link to security policy

Author: sebastianbergmann

composer.json

@@ -17,7 +17,8 @@
         }
     ],
     "support": {
-        "issues": "https://github.com/sebastianbergmann/php-code-coverage/issues"
+        "issues": "https://github.com/sebastianbergmann/php-code-coverage/issues",
+        "security": "https://github.com/sebastianbergmann/php-code-coverage/security/policy"
     },
     "config": {
         "platform": {