How do I feature a post in the intro or outro of my Newsletter?

You can use HTML in the intro and outro blocks, as described here.

If you would like to insert a featured post with an image and more prominent "Read more" button, you can use the following HTML as a guide (this has been tested with Email on Acid):

<!--
This snippet is designed for placement in your Newsletter issue settings under "Intro paragraph" or "Outro paragraph".

Make sure to update all of the placeholder values with your content:
* Image URL as `<img src="IMAGE LINK PLACEHOLDER" />`
* Post title
* Post link TWICE as `<a href="LINK TO POST" ...>` for the title and "Read more" button
* Post content where the "Lorem ipsum..." text is
-->

<table style="color:#2B485C;font-family:sans-serif!important;font-size:15px;line-height:1.4;vertical-align:top;width:100%;border-collapse:collapse;padding:0;border:0;">
 <!-- repeat this <tr> element and its contents for each featured post -->
 <tr style="vertical-align:top;padding:0;border:0;">
 <td style="padding:0;border:0;" valign="top">
 <img src="IMAGE LINK PLACEHOLDER" alt="" style="max-width:250px;width:100%;padding-bottom:20px;border:0;">
 </td>
 <td style="padding:0 0 20px 15px;border:0;max-width:320px;" valign="top">
 <h2 style="font-size:24px;line-height:1.3;margin:0 0 0.75em;padding:0;"><a href="LINK TO POST" style="color:#3AAEE0;text-decoration:none;">Post Title</a></h2>
 <p style="margin:0 0 20px;padding:0;">Post content. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Proin ac erat enim. Vivamus ut aliquam libero. Integer aliquam metus at neque placerat, ac tempor lorem bibendum.</p>
 <a href="LINK TO POST" style="background-color:#3AAEE0;color:#fff;text-decoration:none;padding:5px 15px 7px;">Read&nbsp;more</a>
 </td>
 </tr>
</table>