Update README.md (#1041)

Add command, it's required to run `rails webpacker:install` first if not:
```
webpack binstubs not found.
Have you run rails webpacker:install ?
Make sure the bin directory or binstubs are not included in .gitignore
Exiting!
```

Author: yosefbennywidyo

README.md

@@ -78,6 +78,7 @@ gem 'react-rails'
 ###### Rails 6.x:
 ```
 $ bundle install
+$ rails webpacker:install
 $ rails webpacker:install:react
 $ rails generate react:install
 ```