Add CONTRIBUTING.md template

First, we should add `CONTRIBUTING.md` to *ITKExamples* and *ITKSoftwareGuide*. If a repository has its own version of the helper files, GitHub does not use the one from this repository. *ITK*, *ITKExamples*, and *ITKSoftwareGuide* have `SetupForDevelopment.sh`, and more help for the newcomer to Git / GitHub development.

For other repositories like modules, etc., we could welcome the contributor, point them to the Code of Conduct, briefly describe the standard Git workflow, and provide references to Git / GitHub usage.

Start by reviewing the CONTRIBUTING.md scope for ITKExamples and ITKSoftwareGuide, along with the SetupForDevelopment.sh files mentioned for ITK, ITKExamples, and ITKSoftwareGuide. Then define the shared guidance for modules and other repositories, including the Code of Conduct and standard Git/GitHub workflow; done means the requested CONTRIBUTING.md templates are added in the appropriate repositories.

索引モデルが issue の本文から書いたものです。