Add a README

Closes #66.

README.markdown

@@ -0,0 +1,17 @@
+# Vim Markdown runtime files
+
+This is the development version of Vim's included syntax highlighting and
+filetype plugins for Markdown.  Generally you don't need to install these if
+you are running a recent version of Vim.
+
+One difference between this repository and the upstream files in Vim is that
+the former forces `*.md` as Markdown, while the latter detects it as Modula-2,
+with an exception for `README.md`.  If you'd like to force Markdown without
+installing from this repository, add the following to your vimrc:
+
+    autocmd BufNewFile,BufReadPost *.md set filetype=markdown
+
+## License
+
+Copyright © Tim Pope.  Distributed under the same terms as Vim itself.
+See `:help license`.