At some point, the number of conventions and agreements floating around in your project may become so great that you need to record it somewhere. In order to give such a document legitimacy, make it clear that it is based on mailing list discussions and on agreements already in effect. As you compose it, link to the relevant threads in the mailing list archives, and whenever there's a point you're not sure about, ask again. The document should not contain any surprises: remember, it is not the source of the agreements, it is merely a description of them. Of course, if it is successful, people will start citing it as a source of authority in itself, but that just means it reflects the overall will of the group accurately.
This is the document alluded to in the section called “Developer Guidelines” in Chapter 2, Getting Started. Naturally, when the project is very young, you will have to lay down guidelines without the benefit of a long project history to draw on. But as the development community matures, you can adjust the language to reflect the way things actually turn out.
Don't try to be comprehensive. No document can capture everything people need to know about participating in a project. Many of the conventions a project evolves remain forever unspoken, never mentioned explicitly, yet adhered to by all. Other things are simply too obvious to be mentioned, and would only distract from important but non-obvious material. For example, there's no point writing guidelines like "Be polite and respectful to others on the mailing lists, and don't start flame wars," or "Write clean, readable bug-free code." Of course these things are desirable, but since there's no conceivable universe in which they might not be desirable, they are not worth mentioning. If people are being rude on the mailing list, or writing buggy code, they're not going to stop just because the project guidelines said to. Such situations need to be dealt with as they arise, not by blanket admonitions to be good. On the other hand, if the project has specific guidelines about how to write good code, such as rules about documenting every API in a certain format, then those guidelines should be written down as completely as possible.
A good way to determine what to include is to base the document on the questions that newcomers ask most often, and on the complaints experienced developers make most often. This doesn't necessarily mean it should turn into a FAQ sheet—it probably needs a more coherent narrative structure than FAQs can offer. But it should follow the same reality-based principle of addressing the issues that actually arise, rather than those you anticipate might arise.
If the project is a benevolent dictatorship, or has officers endowed with special powers (president, chair, whatever), then the document is also a good opportunity to codify succession procedures. Sometimes this can be as simple as naming specific people as replacements in case the BD suddenly leaves the project for any reason. Generally, if there is a BD, only the BD can get away with naming a successor. If there are elected officers, then the nomination and election procedure that was used to choose them in the first place should be described in the document. If there was no procedure originally, then get consensus on a procedure on the mailing lists before writing about it. People can sometimes be touchy about hierarchical structures, so the subject needs to be approached with sensitivity.
Perhaps the most important thing is to make it clear that the rules can be reconsidered. If the conventions described in the document start to hamper the project, remind everyone that it is supposed to be a living reflection of the group's intentions, not a source of frustration and blockage. If someone makes a habit of inappropriately asking for rules to be reconsidered every time the rules get in her way, you don't always need to debate it with her—sometimes silence is the best tactic. If other people agree with the complaints, they'll chime in, and it will be obvious that something needs to change. If no one else agrees, then the person won't get much response, and the rules will stay as they are.
Three good examples of project guidelines are the LibreOffice Development guide at wiki.documentfoundation.org/Development, theSubversion Community Guide, at subversion.apache.org/docs/community-guide/, and the Apache Software Foundation governance documents, at apache.org/foundation/how-it-works.html and apache.org/foundation/voting.html. The ASF is really a collection of software projects, legally organized as a nonprofit corporation, so its documents tend to describe governance procedures more than development conventions. They're still worth reading, though, because they represent the accumulated experience of a lot of open source projects.