Add documentation to make the formula package more discoverable and
demonstrate its value as a reusable workflow definition library.
The formula package provides TOML-based workflow definitions with:
- Type inference (convoy, workflow, expansion, aspect)
- Comprehensive validation
- Cycle detection in dependency graphs
- Topological sorting (Kahn's algorithm)
- Ready-step computation for parallel execution
New files:
- doc.go: Package-level godoc with examples and API overview
- README.md: User guide with installation, quick start, and API reference
- example_test.go: Runnable examples for godoc and testing
The package has 130% test coverage (1,200 LOC tests for 925 LOC code)
and only depends on github.com/BurntSushi/toml.
