add rust-mode pure nel readme

README.md

@@ -13,6 +13,7 @@ What's inside
   * [markdown-mode](https://github.com/emacsmirror/markdown-mode)
   * [yaml-mode](https://github.com/yoshiki/yaml-mode)
   * [focus-mode](https://github.com/larstvei/Focus)
+  * [rust-mode](https://github.com/rust-lang/rust-mode)
 
 Update
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