Adding README.md and renaming plugin

plugins/marked2/README.md

+## marked2
+
+Plugin for Marked 2, a previewer for Markdown files on Mac OS X 
+
+### Requirements
+
+ * [Marked 2](http://marked2app.com)
+
+### Usage
+
+ * If `marked` is called without an argument, open Marked
+
+ * If `marked` is passed a file, open it in Marked