described changes wrt. overflows

Road-toward-3.0.0.md

@@ -47,4 +47,10 @@ or
 ```cpp
 json j;
 ss >> j;
-```
+```
+
+## Handling of NaN and INF
+
+- If an overflow occurs during parsing a number from a JSON text, an exception (`std::out_of_range` for the moment, to be replaced by a user-defined exception) is thrown so that the overflow is detected early and roundtripping is guaranteed.
+
+- NaN and INF floating-point values can be stored in a JSON value and are not replaced by null. That is, the basic_json class behaves like double in this regard (no exception occurs). However, NaN and INF are serialized to “null”.