Document how to run Prettier code formatter

Author: strager

docs/CONTRIBUTING.md

@@ -5,6 +5,7 @@
 * All tests pass locally.
 * On CI, all jobs succeed, including the Linux, macOS, Windows, and WASM builds.
 * C++ code is formatted using [clang-format](#clang-format).
+* CSS, HTML, and JavaScript code is formatted using [Prettier](#Prettier).
 
 ## clang-format
 
@@ -26,3 +27,18 @@ quick-lint-js' C++ code is formatted using the [clang-format][] tool, **version
 [clang-format-vscode]: https://marketplace.visualstudio.com/items?itemName=xaver.clang-format
 [clang-format]: https://clang.llvm.org/docs/ClangFormat.html
 [llvm-build-9.0.0]: https://releases.llvm.org/download.html#9.0.0
+
+## Prettier
+
+quick-lint-js' web code (CSS, HTML, and JavaScript) is formatted using the
+[Prettier][] tool. During development, use Prettier in one of two ways:
+
+* In the `plugin/vscode`, `plugin/vscode-lsp`, `wasm`, or `website` directory,
+  install Prettier with `yarn install` then run it with `yarn fmt`. This formats
+  all of quick-lint-js' code in that subproject with one command.
+* Run Prettier in your editor with [Prettier editor
+  integration][Prettier-editor]. You need to run `yarn install` in the
+  subproject's directory in order for editor integration to work.
+
+[Prettier]: https://prettier.io/
+[Prettier-editor]: https://prettier.io/docs/en/editors.html