1. Complete documentation for the contribution must be contained in the contribution's distribution zip file, preferably in a file named "README-addonnamehere.TXT". Using a URL shortcut/link to point to another website for readme and/or install/uninstall documentation is not acceptable.
2. The author should be acknowledged - and a single link to their site is acceptable as is a tasteful request for donations (a single statement about where to send donations is acceptable).
Prior authors/contributors should be acknowledged.
3. Documentation should include clear steps outlining what to do once the contribution has been unzipped, including where to put the unzipped files and how to do any database updates/changes.
4. Documentation should be written with the "newbie" in mind ... ie: Don't make assumptions that the person using the contribution has any more understanding than how to unzip the file. This helps address the lowest-common-denominator skill-set and helps minimize support questions which can cause frustration and time drain on the author to supply support.
5. Documentation should include uninstall instructions in case someone wants to cleanly remove the addon from their store ... without breaking normal operation or leaving old fragments/data behind. This should address any files to be removed, any folders to be removed, any permissions changes to be undone, and any SQL changes/deletions applicable to the addon.