Add README for helix-syntax (#1312)

helix-syntax/README.md

@@ -0,0 +1,13 @@
+helix-syntax
+============
+
+Syntax highlighting for helix, (shallow) submodules resides here.
+
+Differences from nvim-treesitter
+--------------------------------
+
+As the syntax are commonly ported from
+<https://github.com/nvim-treesitter/nvim-treesitter>.
+
+Note that we do not support the custom `#any-of` predicate which is
+supported by neovim so one needs to change it to `#match` with regex.