doc: add simple contributing.md file

CONTRIBUTING.md

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+# Contributing to `nvim-tree.lua`
+
+Thank you for contributing.
+
+## Styling and formatting
+
+Code is formatted using luacheck, and linted using stylua.
+You can install these with:
+
+```bash
+luarocks install luacheck
+cargo install stylua
+```
+
+## Adding new actions
+
+To add a new action, add a file in `actions/name-of-the-action.lua`. You should export a `setup` function if some configuration is needed.
+
+## Documentation
+
+When adding new options, you should declare the defaults in the main `nvim-tree.lua` file.
+Once you did, you should run the `update-default-opts.sh` script which will update the default documentation in the README and the help file.
+
+Documentation for options should also be added, see how this is done after `nvim-tree.disable_netrw` in the `nvim-tree-lua.txt` file.