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 😄

Doc to update: https://docs.gitlab.com/user/project/working_with_projects/#upload-an-avatar-in-project-settings