Add status clarification to README
Subject to change, grammar still evolving.
+However:
+
+* Some neovim / nvim-treesitter folks have been using it
+* At the moment, I don't intend to support "higher level" things like `defn` in the grammar itself. See [this comment](https://github.com/sogaiu/tree-sitter-clojure/issues/15#issuecomment-880729889) for some background.
+
## Prerequisites
* [emsdk](https://emscripten.org/docs/getting_started/downloads.html#installation-instructions) -- emscripten via homebrew seems to work for macos
