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It is likely that if you are reading the Author Guide, you already have a subject in mind. The idea may have come from your own frustrations in trying to get something to work; or maybe you are already writing or have written documentation for other purposes and you want to submit it to the LDP. For example, if you posted a question to a mailing list and it took many posts to resolve the problem -- that might be an incentive to write documentation.
Regardless of how you picked your subject, it is important that the LDP community understand why your documentation should be included in its collection. If someone has requested documentation, or you worked with a mailing list to resolve a problem you should include the details in your proposal to the LDP discuss mailing list. It may help others to understand the need for your specific document.