Update community guidelines and add core team file
Based on the [Moya community guidelines](https://github.com/Moya/contributors) I've updated out `CONTRIBUTING.md` to explicitly explain our "Invite any contributor who has at least one PR merged" philosophy, and added a `CORE_TEAM.md` file which contains a list of core team members. (@geelen, @mxstbr, @philpl, @schwers; in order of addition) I've also already set up an instance of [Aeryn](https://github.com/Moya/Aeryn), which will automatically invite any collaborator that has a PR merged into the main `styled-components` repo to the organisation. I hope that (as with Moya) this makes the `styled-components` longer lasting and removes the core team members as bottlenecks from the evolution of the library.
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