Document how to add GPG key from key server to /etc/apt/keyrings

```bash
gpg --keyserver hkps://keyserver.ubuntu.com --recv-keys 648ACFD622F3D138 && \
gpg --export --armor 648ACFD622F3D138 > yourpubkey.gpg && \
gpg --batch --yes --delete-keys 648ACFD622F3D138
```
Reference: https://superuser.com/questions/1641291/gpg-only-download-a-key-from-a-keyserver