Enable RSS feed auto-discovery

Hi! I noticed you have an RSS feed at https://colah.github.io/rss.xml, but it's not linked from anywhere. Please add the following to `<head>` of each page so that newsreaders could find the feed automatically:

```html
<link rel="alternate" type="application/rss+xml" title="colah's blog" href="rss.xml">
```

Thank you.