Document project avatar feature
The title is a bit cryptic but the feature in question that needs to be documented is that if you have a `logo.png` in the root directory of your git repository it automagically appears as the project's avatar. It might work for other extensions besides `.png` as well, we need to test. This feature almost feels like an easter egg :smile: Doc to update: https://docs.gitlab.com/user/project/working_with_projects/#upload-an-avatar-in-project-settings
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