<p>Markdown is a great tool for formatting plain text into pretty html, but it doesn't turn plain-text links into URLs automatically. Like this one:</p>

<p><a href="http://www.google.com/" rel="noreferrer">http://www.google.com/</a></p>

<p>How do I get markdown to add  tags to URLs when I format a block of text?</p>

This isn't a feature of Markdown -- what you should do is run a post-processor against the text looking for a URL-like pattern. There's a good example in the
Google app engine example code
-- see the
```
AutoLink
```
transform.