Updated: README.md to latest jekyll syntax (#414)
Updated: README.md to latest jekyll syntax (#414)

After installing jekyll-assets, I received the following warning:

  Deprecation: The 'gems' configuration option has been renamed to
  'plugins'. Please update your config file accordingly.

Additionally, the Gemfile uses :jekyll_plugins instead of :plugins

This should be more inline with jekyll version 3.5.2
jeremy/jekyll-assets,jekyll/jekyll-assets,mattbeedle/jekyll-assets,mattbeedle/jekyll-assets,jeremy/jekyll-assets,jeremy/jekyll-assets,jekyll-assets/jekyll-assets,jekyll/jekyll-assets,jeremy/jekyll-assets,jekyll-assets/jekyll-assets,jekyll/jekyll-assets,jekyll/jekyll-assets,mattbeedle/jekyll-assets,mattbeedle/jekyll-assets,jekyll-assets/jekyll-assets