Add Setup section to README

Author: ryanb

README.md

@@ -2,6 +2,20 @@
 
 VSCode support for the [syntax_tree](https://github.com/ruby-syntax-tree/syntax_tree) gem. Available for download in the [VSCode marketplace](https://marketplace.visualstudio.com/items?itemName=ruby-syntax-tree.vscode-syntax-tree).
 
+## Setup
+
+After installing the [syntax_tree](https://github.com/ruby-syntax-tree/syntax_tree) gem and the [VSCode extension](https://marketplace.visualstudio.com/items?itemName=ruby-syntax-tree.vscode-syntax-tree), add the following to your VSCode `settings.json` to format Ruby on save.
+
+```json
+"[ruby]": {
+  "editor.formatOnSave": true,
+  "editor.defaultFormatter": "ruby-syntax-tree.vscode-syntax-tree"
+},
+```
+
+You can find additional config options by searching for "syntax tree" in your VSCode settings.
+
+
 ## Contributing
 
 Bug reports and pull requests are welcome on GitHub at https://github.com/ruby-syntax-tree/vscode-syntax-tree.