Create migration guide from robfig/cron

[CybotTM]

Summary

While the project claims drop-in compatibility with robfig/cron, there's no explicit migration guide documenting any behavioral differences.

Current State

• README mentions it's a fork
• Claims API compatibility
• No formal migration documentation

Required Content

docs/MIGRATION.md

# Migrating from robfig/cron

## Quick Start

1. Update import path:
   \`\`\`go
   // Before
   import "github.com/robfig/cron/v3"
   
   // After  
   import "github.com/netresearch/go-cron"
   \`\`\`

2. Update go.mod:
   \`\`\`
   go get github.com/netresearch/go-cron@latest
   \`\`\`

## Behavioral Differences

### Fixed Bugs (Behavior Changes)

| Issue | robfig/cron | netresearch/go-cron |
|-------|-------------|---------------------|
| TZ= parsing | Panics on edge cases | Returns error |
| Entry.Run() | Bypasses chain | Honors chain wrappers |
| DST spring-forward | Jobs may be skipped | Jobs run immediately |

### New Features

- `Entry.Run()` method for chain-aware execution
- `FakeClock` for deterministic testing
- Heap-based scheduling (performance)

### API Additions (Non-breaking)

- [List any new methods/options]

## Compatibility Notes

- All existing code should work without changes
- Error handling may differ for edge cases
- Performance characteristics improved

## Testing Your Migration

1. Run existing tests
2. Verify timezone handling
3. Test DST transitions if relevant
4. Benchmark if performance-critical

Acceptance Criteria

• Create docs/MIGRATION.md
• Document all behavioral differences
• Document all bug fixes that change behavior
• Add "before/after" code examples
• Link from README
• Review against robfig/cron issues for completeness