Update installation instructions re: unmet peer dependencies (#1496)

See discussion in rails/webpacker#1078

README.md

@@ -94,7 +94,7 @@ gem 'webpacker', '>= 4.0.x'
 yarn add @rails/webpacker@4.0.0-pre.2
 ```
 
-Finally, run following to install Webpacker:
+Finally, run the following to install Webpacker:
 
 ```bash
 bundle
@@ -104,6 +104,11 @@ bundle exec rails webpacker:install
 bundle exec rake webpacker:install
 ```
 
+Optional: To fix ["unmet peer dependency" warnings](https://github.com/rails/webpacker/issues/1078),
+
+```bash
+yarn upgrade
+```
 
 ### Usage