Add contribution guidelines and issue/PR templates

[franz-josef-kaiser]

As a follow up to #119 … details in this GH blog post

• Add .github directory
• Add .gitignore
• Add CONTRIBUTING.md
• Add ISSUE_TEMPLATE.md
• Add PULL_REQUEST_TEMPLATE.md

[dnaber-de]

Do you have already an idea about the content of this guidelines? What are the minimum requirements a tool should fulfill to get listed in wp gear? In order to be as transparent as possible, we have to find and document answers to this question.

[franz-josef-kaiser]

@dnaber-de I just added empty Guideline files. We can discuss the general idea on Slack and then proceed here so it does not overflow.