Add <link> tag to RSS feed on posts root

This is a simple change that builds off of the larger RSS work in artisan-tattoo/artisan_assistant#6. It adds a <link> tag to help browsers and feed readers auto-detect the feed. Please disregard if this isn’t necessary or within the project’s best practices.

### How to test

A simple acceptance test to run between `master` and the PR, asserting that there’s at least one `application/rss+xml` link `¯\_(ツ)_/¯`:

```ruby
bundle exec ruby << RB_TEST
require 'minitest/autorun'
require 'nokogiri'
require 'open-uri'

class TestPostsIndexPage < MiniTest::Test
  def test_includes_rss_link
    posts_page = open('http://localhost:3000/posts')
    doc = Nokogiri::HTML(posts_page)
    assert doc.xpath("//link[@type='application/rss+xml']").count > 0,
          "Posts index page should include auto-detectable RSS <link>"
  end
end
RB_TEST
```

Thanks!