add news item

Author: czeildi

NEWS.md

@@ -340,6 +340,8 @@ See [#2611](https://github.com/Rdatatable/data.table/issues/2611) for details. T
 
 20. `forderv` could segfault on keys with long runs of identical bytes (e.g., many duplicate columns) because the single-group branch tail-recursed radix-by-radix until the C stack ran out, [#4300](https://github.com/Rdatatable/data.table/issues/4300). This is a major problem since sorting is extensively used in `data.table`. Thanks @quantitative-technologies for the report and @ben-schwen for the fix.
 
+21. `[` keeps existing key(s) when new columns added before them instead of incorrectly setting a new column as key, [#7364](https://github.com/Rdatatable/data.table/issues/7364). Thanks @czeildi for the bug report and the fix.
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 ### NOTES
 
 1. The following in-progress deprecations have proceeded: