Alternately, you can use this tool to generate one:
http://tools.trepmal.com/wp-readme-generator/

A readme.txt is needed so your plugin will display correctly in our repository, but also so we can make sure you're providing the users with all the information they need before they install your plugin. Our goal with this is to make sure everyone knows what they're installing and what they need to do before they install it. No surprises :)

This is especially important if your plugin is making calls back to your own servers. For the most part, we do not permit offloading of images or code, however in the case where you are providing a service (like Disqus or Akismet or Twitter), we permit it. The catch is you have to actually explain this to the layman in your readme, so they know where data is going.

Keep in mind, a readme.MD is not the same as a readme.TXT - the formatting for markup is different.

WordPress includes its own version of jquery, which has been rigorously tested with WP and many of the most common plugins. In order to provide the best compatibility and experience for our users, we ask that you not package your own (especially not an older version) and instead use wp_enqueue_script() to pull in WordPress’s version.

Please review http://codex.wordpress.org/Function_Reference/wp_enqueue_script and update your plugin accordingly.

Keep in mind: Offloading the default jquery and other scripts to Google is similarly disallowed. If your code doesn't work with the built-in jQuery, it's most likely a noconflict issue. If you can't guess, we -really- want you to use our jquery, and if you can't, we need to know why so we can fix things for everyone.

Please send a link so the completed plugin can be downloaded. Alternately you can reply to this and send a .zip or .rar file. As this is for a new plugin request, please reply within seven days of this email or we will reject your plugin.

All plugins are expected to be submitted with the code complete and ready to go, including a valid readme.txt file. Could you please reply with a link so the plugin can be downloaded? Alternately you can reply to this and attach your .zip or .rar file.

Please reply with 7 days. If a reply is not received the submission will be rejected and you will have to submit the plugin again, with code, should you want your plugin hosted.

Thanks.

All plugins are expected to be submitted with the code complete and ready to go, including a valid readme.txt file. Could you please reply with a link so the plugin can be downloaded? Alternately you can reply to this and attach your .zip or .rar file.

Please reply with 7 days. If a reply is not received the submission will be rejected and you will have to submit the plugin again, with code, should you want your plugin hosted.

Thanks.