limit formatOnSave option to JS/TS files, add docs re: prettier

docs/in-depth/prettier.md

@@ -0,0 +1,19 @@
+# Prettier
+
+[Prettier](https://prettier.io/) is an automatic code formatter which supports various languages, including TypeScript. It also has extensions for various text editors like [VSCode](https://github.com/prettier/prettier-vscode), [Atom](https://github.com/prettier/prettier-atom), and even [Vim](https://github.com/prettier/vim-prettier). If you have installed these extensions, it will use Prettier's service to automatically format your code after saving.
+
+If you would rather not use Prettier instead, you can easily disable it too. In VSCode, open `.vscode/settings.json`, then change the `"editor.formatOnSave"` option to `false`:
+
+```json
+{
+  "[json]": {
+    "editor.formatOnSave": false
+  },
+  "[javascript]": {
+    "editor.formatOnSave": false
+  },
+  "[typescript]": {
+    "editor.formatOnSave": false
+  }
+}
+```