Refresh README.md, CONTRIBUTING.md (#11413)
This was prompted by #11403
Changes include:
- General streamlining of markdown
- Remove outdated references, e.g. to Python 2 / how to install Python 3
- Mention the "type school" discussions repo, typing-sig mailing list
- Remove roadmap, since there isn't really a mypy core team anymore
- More concretely link to "good first issue" page, etc. Hopefully this
is a better experience than the somewhat common case of gitter posts
like "I know Python, please help me contribute" that don't
really go anywhere
- Consolidate contributing information in CONTRIBUTING.md
- Remove issue tracker conventions, since the template does a pretty
good job and the rest is either common sense or never looked at
