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
+```

scripts/release/steps/update-version.js

@@ -15,6 +15,9 @@ async function bump({ version }) {
   processFile(".github/ISSUE_TEMPLATE/integration.md", (content) =>
     content.replace(/^(- Prettier Version: ).*?$/m, `$1${version}`)
   );
+  processFile("docs/install.md", (content) =>
+    content.replace(/^(npx prettier@)\S+/m, `$1${version}`)
+  );
 
   // Update unpkg link in docs
   processFile("docs/browser.md", (content) =>