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.

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.

Usually plugins will include wp-config.php or wp-load.php in order to gain access to core WordPress functions, but there are much better ways to do this.

It's best if you tie your processing functions (the ones that need but don't have access to core functions) into an action hook, such as "init" or "admin_init".

Please consult the Plugins API reference for more information: http://codex.wordpress.org/Plugin_API

When you've corrected your code, reply to this email with the updated code attached, or provide a link to the new code.