mention option

Author: MichaelChirico

NEWS.md

@@ -8,7 +8,7 @@
 
 1. `data.table(x=1, <expr>)`, where `<expr>` is an expression resulting in a 1-column matrix without column names, will eventually have names `x` and `V2`, not `x` and `V1`, consistent with `data.table(x=1, <expr>)` where `<expr>` results in an atomic vector, for example `data.table(x=1, cbind(1))` and `data.table(x=1, 1)` will both have columns named `x` and `V2`. In this release, the matrix case continues to be named `V1`, but the new behavior can be activated by setting `options(datatable.old.matrix.autoname)` to `FALSE`. See point 5 under Bug Fixes for more context; this change will provide more internal consistency as well as more consistency with `data.frame()`.
 
-2. `t1 - t2`, where both `t1` and `t2` are `IDate`, will eventually have class `difftime`, consistent with the case where `t1` and `t2` are both `Date`. See point 17 under Bug Fixes for more context.
+2. `t1 - t2`, where both `t1` and `t2` are `IDate`, will eventually have class `difftime`, consistent with the case where `t1` and `t2` are both `Date`. You can activate the new behavior by setting `options(datatable.old.diff.time = FALSE)`. See point 17 under Bug Fixes for more context.
 
 ### NEW FEATURES