Mention MIVOT in the RESOURCE section

[lmichel]

Add a small paragraph in the RESOURCE section telling that:

• RESOURCE@type=meta can host MIVOT blocks
• What are MIVOT annotations
• Why this has no impact on the VOTable schema

This is just informative, no need to mention it in the CHANGE section

[tomdonaldson]

This informational section seems very helpful to me.

The current process (for better or worse) has us building new working drafts off of the master branch, so I will switch this merge to go there.