Add CONTRIBUTING.md

[serin-delaunay]

• Refer to https://github.com/Wangscape/Wangscape/blob/master/CONTRIBUTING.md.
• Explain the purpose of the repository.
• Mention AppVeyor's 60 minute time limit.
• Suggest turning off other examples in appveyor.yml if using a PR to test output.
• Mention where fixed maps should be placed.
• Mention that copyright info is required for base tiles.
• Mention the license requirements for configurations (see Add license for configurations #2).
• ...