chore(readme): note that snipmate contributions should be preferred

since UltiSnips supports both

Author: sanjayankur31

README.md

@@ -169,6 +169,9 @@ on the fly adding those.
 Currently all snippets from UltiSnips have been put into `/UltiSnips` - some work
 on merging should be done (dropping duplicates etc). Also see engines section above.
 
+Since UltiSnips supports both UltiSnip and vim-snipmate snippets, when
+contributing fixes/new snippets, please prefer adding vim-snipmate snippets.
+
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