Make it available on Firebase Open Source site

[dlackty]

First of all, thank you for making this nice library! Codable is really important for modern iOS development.

Have you ever heard about https://firebaseopensource.com? This is an official collection of Firebase's OSS projects and it's a pity that CodableFirebase isn't listed.

I tried to add a project metadata file as described in firebase/firebaseopensource.com. Please let me know how you think, and hope this helps!