MathML Core meeting agenda addition

Rob is about to upstream code for radicals and needs a resolution on mroot
and number of children. From Rob:

> The issue is about mroot. The spec says that base and index can only be
> one child. However last year we experimented with the index consisting of
> more than one child, and this is the current situation in the internal
> branch.


I have added this as the first item for the meeting that is coming up soon.