Inconsistency in README regarding xtra

[leventov]

In readme, there is this line: "Attempts to update or insert with xtra fields are ignored." As it stands, unconditional, it contradicts the example just above it, where a row is inserted with xtra, and the example just below it, where the attempt to update with xtra raises a error (that is, not the no-op, as the line "... are ignored" implies).