add dev gem to generate erd

Gemfile

@@ -81,6 +81,8 @@ group :development do
   gem 'solargraph'
   # Use Pry as your rails console
   gem 'pry-rails', '~> 0.3.9'
+  # Automatically generate an entity-relationship diagram (ERD) for your Rails models.
+  gem 'rails-erd'
 end
 
 group :test do