mention npx in install docs

docs/install.md

@@ -20,3 +20,9 @@ npm install --global prettier
 ```
 
 > We recommend pinning an exact version of prettier in your `package.json` as we introduce stylistic changes in patch releases.
+
+If you use `npx` to run Prettier, the version should be pinned like this:
+
+```bash
+npx prettier@2.0.1 . --write
+```