Add developer marketing plan

Comprehensive strategy for promoting javascript-proxy-headers:
- Package registry publishing (npm, GitHub Packages, JSR)
- Awesome lists PRs (awesome-nodejs, awesome-web-scraping)
- Library documentation PRs (axios, got, undici, node-fetch, superagent)
- Direct maintainer outreach strategy
- Content marketing (blog posts, videos)
- Community engagement (Stack Overflow, Reddit, Discord)
- SEO and discoverability optimization
- Metrics and tracking plan

Made-with: Cursor

Author: proxymeshai