Add a TODO

Author: MichaelChirico

src/data.table.h

@@ -17,6 +17,7 @@
 #if R_VERSION < R_Version(3, 4, 0)
 #  define SET_GROWABLE_BIT(x)  // #3292
 #endif
+// TODO: remove the `R_SVN_VERSION` check when R 4.5.0 is released (circa Apr. 2025)
 #if R_VERSION < R_Version(4, 5, 0) || R_SVN_REVISION < 86702
 #  define isDataFrame(x) isFrame(x) // #6180
 #endif