Add CONTRIBUTING.md

[tomotomo]

Popular open source projects have CONTRIBUTING.md for contributors. When I want to contribute iotit, it will be useful very much. I gave up to add Raspberry Pi 3 B+ support because I couldn't understand your workflow. (To tell the truth, I couldn't find a code to fix it)

Please show me contributing guideline.

examples:

• https://github.com/nodejs/node/blob/master/CONTRIBUTING.md
• https://github.com/Homebrew/homebrew-cask/blob/master/CONTRIBUTING.md
• https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md
• https://github.com/golang/go/blob/master/CONTRIBUTING.md