Add Markdown markup classifier (#45)

A package that does text processing of Markdown files can reference this classifier.

Author: mondeja

src/trove_classifiers/__init__.py

@@ -728,6 +728,7 @@
     "Topic :: Text Processing :: Markup",
     "Topic :: Text Processing :: Markup :: HTML",
     "Topic :: Text Processing :: Markup :: LaTeX",
+    "Topic :: Text Processing :: Markup :: Markdown",
     "Topic :: Text Processing :: Markup :: SGML",
     "Topic :: Text Processing :: Markup :: VRML",
     "Topic :: Text Processing :: Markup :: XML",