Re-organize docs

[ianlewis]

We need to organize docs by workflow.

• README.md - base readme for the project. Links to docs for individual workflows
  • builders/go/README.md - Doc for Go builder.
  • builders/slsa/README.md - Doc for provenance-only workflow
  • etc.

[laurentsimon]

We also need to document the release process in a RELEASE.md. See #74