Add the community governance document

[oleg-nenashev]

This pull request documents the current status of the community governance with the following intensions:

• Document contributor roles, especially what "contributor" means
• Be explicit that the component maintainers retain executive power in their components (important for hosting)
• Document the decision making process, encourage consensus there
• Formally document the BDFL role of @tomakehurst
• Document what public roadmap means and (not) commits to

References

• https://github.com/wiremock/community/issues/19
• https://github.com/wiremock/community/issues/7