Document Statlib's design-first contribution workflow

[zixiaowang17]

Summary

• turn the repository README into a useful entry point for Statlib
• document the design-before-code workflow for substantial formalization projects
• explain why naming, representation, code conventions, and Mathlib compatibility should be agreed in an issue before large implementations begin
• clarify that design decisions can be revisited in the issue during implementation
• mark roadmap/README.md as an optional design-document checklist rather than a required format
• add project links, current areas, build instructions, contribution expectations, and license information

Workflow described

1. Open an issue with the mathematical source, scope, definitions, results, and existing foundations.
2. Discuss and agree on the design specification in that issue.
3. Implement after the direction is clear, returning to the issue when design choices change.
4. Open a reviewable pull request linked to the issue.

Small fixes and narrowly scoped lemmas are explicitly exempted from the full design-document process.

Verification

• checked all linked Statlib website pages
• git diff --check