Update README for installation steps

[thejimbirch]

For best practices, this:

1. Moves custom module and theme to the correct folders.
2. Installs Drupal.
3. Creates User 2 and 3 with admin and editor roles.
4. Blocks User 1.
5. Applies recipes.

# Copy custom modules to the correct place.
cp -R web/themes/contrib/saplings_child web/themes/custom/
cp -R web/modules/contrib/saplings_custom web/modules/custom/

# Install Drupal.
fin drush site-install minimal -y --site-name="Saplings" --account-name=admin [email protected] [email protected]

# Create a new user and block user 1.
fin drush user-create exampleadmin [email protected]
fin drush user-block admin -y

# Apply recipes.
fin recipe-apply saplings
fin recipe-apply saplings-events

# Add role to user 2.
fin drush user-add-role administrator exampleadmin

# Create a Content editor, User 3.
fin drush user-create exampleeditor [email protected]
fin drush user-add-role content_editor exampleeditor

# Clear cache.
fin drush cr