Add markdown to suggest instead of require

Author: barryvdh

composer.json

@@ -8,6 +8,9 @@
     "require": {
         "php": ">=5.3.3"
     },
+    "suggest": {
+        "dflydev/markdown": "1.0.*"
+    },
     "autoload": {
         "psr-0": {"phpDocumentor": ["src/"]}
     },