This article is not meant as a set of rules which you have to follow by any means. Its more like a set of guidelines, which are meant to make your addons a bit more compatible to other addons and also make your life a bit more easy when dealing with other developers.
1. The most important rule: Don't forget writing a manual. Even if you think using your addon is the most trivial thing in the world - it usually isn't. A few lines about how to install and how to use it are sometimes enough, but they have to be there. Also make sure that somebody is able to read your manual. You have many (free) tools for creating good looking PDF format manuals. Use them. Don't put your documentation in orbiters root folder. There is a dedicated folder "doc" around for this task. Don't name your manual "readme.txt". This file is often replaced soon by other addons.
2. Don't violate the copyrights of other developers. You sure don't want that this happens to you, and in most cases in our large addon population, you can find very well done addons, which just want to be credited properly in your own credits. Does it really harm somebody to write "These meshes had been done by x y z"? No!
4. Don't overwrite files from orbiters base distribution. Its OK to include a line about how to put a base on a planet and let the player himself figure it out how to do this. But very often such modifications damage the base orbiter installation and make it impossible to run the default scenarios. You don't need to do this, so avoid it. When you replace a file from orbiter, make sure the addon works with all default scenarios.
5. Don't overwrite files from other addons. Its almost impossible to check the meshes or textures of all released addons, but often addons overwrite files even of popular addons. Like orbiters basic files, other addons may no longer function if you start overwriting their files. The same may also happen to your addons - do you really want to risk this trouble?
6. Bug reports are good. Don't get angry if somebody reports only the bugs of your addon instead of writing a good critique. You did not find these bugs before relasing it, so its your own fault that somebody can even post the bug reports. Also such bug reports are posted for improving your addon, because somebody is really interested into it.
7. Use names and filenames that make sense. Thats normally no big problem, but especially when two addons simulate the same historic vehicle, it may happen that both use the same parts and filenames.