Add README with hosts table and instructions

README.md

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+Public SSH keys for iscg infra hosts.
+
+Used by `nixos-iscg` flake via sha256-pinned fetchurl on the
+`https://git.iscg.dev/mikl.keys` endpoint.
+
+## Hosts
+
+| Host | Key file | Purpose |
+|------|----------|---------|
+| poppy | poppy.pub | macOS laptop (main) |
+| poppy | poppy-legacy.pub | macOS laptop (fallback) |
+| lilac | lilac.pub | Android phone |
+
+## Adding a key
+
+1. Generate: ssh-keygen -t ed25519 -C "<host>@<host>" -f ~/.ssh/<host> -N ""
+2. Copy .pub here
+3. Update table above
+4. Add to forgejo account Settings -> SSH Keys