Update documentation/other docs, enforce, SPDX license
Rough list of tasks
- split (and update) README - BUILDING, CONTRIBUTING, other
- CONTRIBUTING - pick any bits from ^^, fold docs/code-style.txt, document clang-format, document better sob (add in-tree copy of cert?)
- Add ci-fairy hooks for access, sob, other - see libinput for inspiration
- Document the use of SPDX, update the whole project