Add docs for pnpm (#14455)

docs/install.md

@@ -18,6 +18,12 @@ npm install --save-dev --save-exact prettier
 yarn add --dev --exact prettier
 ```
 
+<!--pnpm-->
+
+```bash
+pnpm add --save-dev --save-exact prettier
+```
+
 <!--END_DOCUSAURUS_CODE_TABS-->
 
 Then, create an empty config file to let editors and other tools know you are using Prettier:
@@ -61,6 +67,14 @@ yarn prettier . --write
 
 > What is `yarn` doing at the start? `yarn prettier` runs the locally installed version of Prettier. We’ll leave off the `yarn` part for brevity throughout the rest of this file!
 
+<!--pnpm-->
+
+```bash
+pnpm exec prettier . --write
+```
+
+> What is `pnpm` doing at the start? `pnpm prettier` runs the locally installed version of Prettier. We’ll leave off the `pnpm` part for brevity throughout the rest of this file!
+
 <!--END_DOCUSAURUS_CODE_TABS-->
 
 `prettier --write .` is great for formatting everything, but for a big project it might take a little while. You may run `prettier --write app/` to format a certain directory, or `prettier --write app/components/Button.js` to format a certain file. Or use a _glob_ like `prettier --write "app/**/*.test.js"` to format all tests in a directory (see [fast-glob](https://github.com/mrmlnc/fast-glob#pattern-syntax) for supported glob syntax).
@@ -118,6 +132,15 @@ For example, you can do the following to have Prettier run before each commit:
 
    > If you use Yarn 2, see https://typicode.github.io/husky/#/?id=yarn-2
 
+   <!--pnpm-->
+
+   ```bash
+   pnpm add --save-dev husky lint-staged
+   pnpm exec husky install
+   npm pkg set scripts.prepare="husky install"
+   pnpm exec husky add .husky/pre-commit "pnpm exec lint-staged"
+   ```
+
    <!--END_DOCUSAURUS_CODE_TABS-->
 
 2. Add the following to your `package.json`: