Document automatic_img_size/alt features
Document automatic_img_size/alt features

Fixed #363

jekyll/jekyll-assets,mattbeedle/jekyll-assets,mattbeedle/jekyll-assets,jekyll/jekyll-assets,mattbeedle/jekyll-assets,jeremy/jekyll-assets,jekyll-assets/jekyll-assets,jekyll-assets/jekyll-assets,jekyll-assets/jekyll-assets,jekyll/jekyll-assets,jekyll/jekyll-assets,mattbeedle/jekyll-assets,jeremy/jekyll-assets,jeremy/jekyll-assets,jeremy/jekyll-assets