Recommend pinning prettier version (#2920)

* Recommend pinning prettier version

* Change from -E to --exact/--save-exact

README.md

@@ -203,7 +203,7 @@ Prettier does nothing to help with those kind of rules. They are also the most i
 Install:
 
 ```
-yarn add prettier --dev
+yarn add prettier --dev --exact
 ```
 
 You can install it globally if you like:
@@ -215,9 +215,14 @@ yarn global add prettier
 *We're using `yarn` but you can use `npm` if you like:*
 
 ```
-npm install [--save-dev|--global] prettier
+npm install --save-dev --save-exact prettier
+# or globally
+npm install --global prettier
 ```
 
+> We recommend pinning an exact version of prettier in your `package.json`
+> as we introduce stylistic changes in patch releases.
+
 ### CLI
 
 Run Prettier through the CLI with this script. Run it without any