Add install generators options that were missing in the documentation.

[soartec-lab]

Hi, first of all, thank you for making this awesome gem.

When I used the generator, I noticed a discrepancy between the options listed in the documentation and the options that were actually enabled.

This PR adds the following to the documentation, which are valid options for the current generator:

• directory
• skip-mutation-root-type
• api

current excute resule

$ pwd
spec/dummy

$ bundle exec rails g graphql:install --help

Usage:
  bin/rails generate graphql:install [options]

Options:
  [--skip-namespace], [--no-skip-namespace]                    # Skip namespace (affects only isolated engines)
                                                               # Default: false
  [--skip-collision-check], [--no-skip-collision-check]        # Skip collision check
                                                               # Default: false
  [--directory=DIRECTORY]                                      # Directory where generated files should be saved
                                                               # Default: app/graphql
  [--schema=SCHEMA]                                            # Name for the schema constant (default: {app_name}Schema)
  [--skip-keeps], [--no-skip-keeps]                            # Skip .keep files for source control
                                                               # Default: false
  [--skip-graphiql], [--no-skip-graphiql]                      # Skip graphiql-rails installation
                                                               # Default: false
  [--skip-mutation-root-type], [--no-skip-mutation-root-type]  # Skip creation of the mutation root type
                                                               # Default: false
  [--relay], [--no-relay]                                      # Include installation of Relay conventions (nodes, connections, edges)
                                                               # Default: true
  [--batch], [--no-batch]                                      # Include GraphQL::Batch installation
                                                               # Default: false
  [--playground], [--no-playground]                            # Use GraphQL Playground over Graphiql as IDE
                                                               # Default: false
  [--api], [--no-api]                                          # Preconfigure smaller stack for API only apps
                                                               # Default: false

Runtime options:
  -f, [--force]                    # Overwrite files that already exist
  -p, [--pretend], [--no-pretend]  # Run but do not make any changes
  -q, [--quiet], [--no-quiet]      # Suppress status output
  -s, [--skip], [--no-skip]        # Skip files that already exist

Install GraphQL folder structure and boilerplate code

Generator souce code

https://github.com/rmosolgo/graphql-ruby/blob/master/lib/generators/graphql/install_generator.rb

[rmosolgo]

Thanks for adding these!