Update README

Author: kmaasrud

README.md

@@ -42,6 +42,7 @@ In Lua:
 require('monokai').setup {}
 require('monokai').setup { palette = require('monokai').pro }
 require('monokai').setup { palette = require('monokai').soda }
+require('monokai').setup { palette = require('monokai').ristretto }
 ```
 
 ## Customization