Document dependency system in Omni/Bild/README.md

Add comprehensive documentation for the dependency pinning system.

Content to add:

1. Decision tree for adding dependencies:

• Is it in nixpkgs?
• Yes, need specific version? → Add to Sources.json (auto-override=true implied)
• Yes, default version OK? → Just use it directly
• No? → Add to Sources.json + create Deps/*.nix derivation + set auto-override=false

2. Workflow examples: `bash # Update all deps deps update

# Update specific dep to branch deps update nixpkgs -b nixos-24.11

# Add new GitHub dep deps add owner/repo

# Review changes git diff Omni/Bild/Sources.json

# Rollback git checkout Omni/Bild/Sources.json `

3. Explain the file structure:

• Sources.json = lockfile (pinned revs + hashes)
• Sources.nix = niv boilerplate for fetching
• Functions.nix = helpers (overridePinnedDeps, overrideSource)
• Deps.nix = system package overlays
• Deps/*.nix = custom derivations for new packages
• Python.nix, Haskell.nix = language-specific overlays

4. Explain auto-override field purpose

Files: Omni/Bild/README.md