Hmm, I guess these point should be obvious, but may as well say it:
-Explain what the script is supposed to do. Sometimes it isn't always obvious from the title of the script. Sometimes people don't know what a certain acronym/military term means. Just explain it the best you can, and don't assume the reader knows anything.
-Point out any bugs/problems in the script, so beta testers don't waste their time pointing out known flaws to you.
A couple things that I also like when people do them:
-Give some ideas on how the script can be used. Especially any uses that might not be immediately obvious. This will be more important for certain scripts, where their purpose is less obvious.
-List any future improvements you plan to make to the scripts. Ask for suggestions if you are short on ideas.
-If there is anything SPECIFIC that you want tested about the scripts, point that out. For example, if you aren't sure if the script works in MP, or if you think problems may occur when using the script in a certain situation, or if you want to know how it seems to affect the AI, etc.
One last, yet important practice that unfortunately gets overlooked every so often:
-When the script has reached a point of completion, submit it to the editor's depot!
There have been more than a few great scripts that have been posted here, worked on by the author until they are basically 'finished', yet never submitted to the ed depot. Also keep in mind that you can always update a script after you send it to the depot.