Add stewardship incubation text

[ericherman]

Co-authored-by: Felix Faassen felix@publiccode.net

Description

Add text to README and CONTRIBUTING to mention the roles of the Product
and Technical Steering groups and the Foundation.

Signalen#108

Checklist

• Keep the PR, and the amount of commits to a minimum
• The commit messages are meaningful and descriptive
• The change/fix is well documented, particularly in hard-to-understand areas of the code / unit tests
• Documentation has been updated if needed
• Are there any breaking changes in the code, if so are they discussed and did the team agreed to these changes
• Check that the branch is based on master and is up to date with master
• Check that the PR targets master
• There are no merge conflicts
• There are no conflicting Django migrations

How has this been tested?

(Not applicable)

• Provided unit tests that will prove the change/fix works as intended
• Tested the change/fix locally and all unit tests still pass
• Code coverage is at least 90% (the higher the better)
• No iSort issues are present in the code
• No Flake8 issues are present in the code

[tcoenen]

Akkoord!