Non-normative sections in the metadata spec

[attilamolnar]

Sections to work on:

• Security considerations
• Implementation considerations
• Moderation considerations

[jwheare]

These will probably take a very similar form to the message-tags-3.3 sections: https://github.com/ircv3/ircv3-specifications/blob/8eb2bd8b21f1216a21ffd88eb9b7cf94057c27af/core/message-tags-3.3.md#security-considerations