Add issue template

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,10 @@
+<!--
+This repository includes the Tree-sitter parser for Elixir,
+whereas editor-specific integrations are separate projects.
+
+If you're running into Tree-sitter related issues in your editor,
+please report it on the respective repository (likely an editor
+plugin responsible for Tree-sitter integration).
+
+For example, when using Neovim, go to https://github.com/nvim-treesitter/nvim-treesitter
+-->