Update bundle analyzer readme (#27507)

Follow-up to #27463 this adds a note about installing as `devDependency` instead of changing the default install command as the plugin being a `devDependency` might cause issues unless wrapped in a `process.env` check. 


## Documentation / Examples

- [x] Make sure the linting passes

packages/next-bundle-analyzer/readme.md

@@ -5,15 +5,17 @@ Use `webpack-bundle-analyzer` in your Next.js project
 ## Installation
 
 ```
-npm install -D @next/bundle-analyzer
+npm install @next/bundle-analyzer
 ```
 
 or
 
 ```
-yarn add --dev @next/bundle-analyzer
+yarn add @next/bundle-analyzer
 ```
 
+Note: if installing as a `devDependency` make sure to wrap the require in a `process.env` check as `next.config.js` is loaded during `next start` as well.
+
 ### Usage with environment variables
 
 Create a next.config.js (and make sure you have next-bundle-analyzer set up)