add README

README.md

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+# vim-zscript
+
+Highlighting for GZDoom's *ZScript* scripting language in Vim.
+
+## Installation
+
+Use [Vim's built-in package manager](http://vimhelp.appspot.com/repeat.txt.html#packages).
+
+## License
+
+[CC0 public domain](https://creativecommons.org/publicdomain/zero/1.0/legalcode).