Add `dev-setup` documentation

dev-setup.md

@@ -0,0 +1,25 @@
+# Development Setup
+
+## Lint / format extensions
+
+### VSCode
+Install Prettier, ESLint and Svelte extensions.
+
+in User `settings.json` (`cmd + shift + p` and search for Open User Settings JSON) add the following:
+
+```json
+{
+  "editor.formatOnSave": true,
+  "[javascript][typescript][css]": {
+    "editor.defaultFormatter": "esbenp.prettier-vscode",
+    "editor.tabSize": 2,
+    "editor.formatOnSave": true
+  },
+  "[svelte]": {
+    "editor.defaultFormatter": "svelte.svelte-vscode",
+    "editor.tabSize": 2
+  },
+  "svelte.enable-ts-plugin": true,
+  "eslint.validate": ["javascript", "svelte"]
+}
+```