Added Markdown icon. (#1310)

_data/simple-icons.json

@@ -1480,6 +1480,11 @@
             "hex": "FF1E0D",
             "source": "http://www.makerbot.com/makerbot-press-assets"
         },
+        {
+            "title": "Markdown",
+            "hex": "000000",
+            "source": "https://github.com/dcurtis/markdown-mark"
+        },
         {
             "title": "Marketo",
             "hex": "5C4C9F",

icons/markdown.svg

@@ -0,0 +1 @@
+<svg role="img" viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg"><title>Markdown icon</title><path d="M22.269 19.385H1.731a1.73 1.73 0 0 1-1.73-1.73V6.345a1.73 1.73 0 0 1 1.73-1.73h20.538a1.73 1.73 0 0 1 1.73 1.73v11.308a1.73 1.73 0 0 1-1.73 1.731zm-16.5-3.462v-4.5l2.308 2.885 2.307-2.885v4.5h2.308V8.078h-2.308l-2.307 2.885-2.308-2.885H3.461v7.847zM21.231 12h-2.308V8.077h-2.307V12h-2.308l3.461 4.039z"/></svg>