docs(readme): options.metadataSubscribers

[jameswomack]

Please Read the CONTRIBUTING Guidelines
In particular the portion on Commit Message Formatting

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

Long-awaited documentation for the metadataSubscribers option merged here

What is the current behavior? (You can also link to an open issue here)
It's undocumented

What is the new behavior?
It's documented

Does this PR introduce a breaking change?

• Yes
• No