Make it more obvious which Gemfile groups to use

Some people missing the fact that the code example has
factory_bot_rails in both the test and development groups.
Adding it in regular text as well makes it a bit more obvious.

README.md

@@ -27,7 +27,7 @@ Gem:
 
 ## Configuration
 
-Add `factory_bot_rails` to your Gemfile:
+Add `factory_bot_rails` to your Gemfile in both the test and development groups:
 
 ```ruby
 group :development, :test do