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)_/¯`:

```
bundle exec rake test test/controllers/posts_controller_test.rb
```

Thanks!