Tidyverse also has a contribution guide on its project site here: https://www.tidyverse.org/contribute/. It might make sense to have this in just one place for your teams projects, rather than having one for each project.
Neither of these mention a license. I would add something like "Your contribution is licensed under GPL 3.x".
It's a good idea to maintain a unified style and structure to the code. This helps development in the future. You can mention these in the contribution guide, but be mindful that these can increase the barrier to contribute. So in a smaller project you might leave these out and mention them when reviewing a pull request or do them after merging.
The more you add to the guide, the less work you need to do yourself (because the contributor is expected to do it).
Here is an example from tidyverse: https://dplyr.tidyverse.org/CONTRIBUTING.html
Tidyverse also has a contribution guide on its project site here: https://www.tidyverse.org/contribute/. It might make sense to have this in just one place for your teams projects, rather than having one for each project.
Neither of these mention a license. I would add something like "Your contribution is licensed under GPL 3.x".
It's a good idea to maintain a unified style and structure to the code. This helps development in the future. You can mention these in the contribution guide, but be mindful that these can increase the barrier to contribute. So in a smaller project you might leave these out and mention them when reviewing a pull request or do them after merging.
The more you add to the guide, the less work you need to do yourself (because the contributor is expected to do it).