nbtree README: Add note about latestRemovedXid.

Point out that index tuple deletion generally needs a latestRemovedXid
value for the deletion operation's WAL record.  This is bound to be the
most expensive part of the whole deletion operation now that it takes
place up front, during original execution.

This was arguably an oversight in commit 558a9165e08, which moved the
work required to generate these values from index deletion REDO routines
to original execution of index deletion operations.

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