When interacting with the codebase, leave it as tidy as possible (pretty much a code-version of "leave the campground at least as clean as you found it."), and always produce some sort of documentation on changes or on interesting things you've seen.
The codebase I'm working with has 150+ contributors continuously committing and keeps growing as we add new features and refactor legacy code. Not a day goes by where I don't see a piece of code I don't understand; without the documentation I produce, I would hit the same wall all the time and would have to repeat the research process. Fortunately, we've got an internal wiki going, and I'm looking forward to merging some of my notes in there so encourage a greater documentation culture amongst my peers.
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