Add announcement of Fastify's involvement in GitHub's OSS Fund

### Prerequisites

- [x] I have written a descriptive issue title
- [x] I have searched existing issues to ensure the issue has not already been raised

### Issue

Fastify was selected to participate in [GitHub Secure Open Source Fund](https://github.blog/open-source/maintainers/securing-the-supply-chain-at-scale-starting-with-71-important-open-source-projects/#web-servers-networking-and-gateways-%f0%9f%96%a5%ef%b8%8f) and as part of that myself and @Eomm have been going on a tear locking things down.

Could be worth a post stating what we've been doing?