Suggestions to improve CONTRIBUTING.md

[Malvoz]

• [x] There should be a link to CONTRIBUTING.md from README.md
• [ ] Change heading "Contributing to MapML" (remove/replace "MapML", as this is not the spec repo)
• [ ] Add build instructions, etc(?).
• [ ] Mention that we attempt to follow the TAG Advice for polyfill authors and TAG Guidelines for web compatible components
• [ ] Mention that we must design with accessibility in mind (particularly, conform to WCAG). Add one or more resources to aid contributors in this matter?

Any takeaways from UCR/CONTRIBUTING.md?

This issue relates to https://github.com/Maps4HTML/chat/issues/22.

Feel free to discuss the above points or add new items to the list.