🚧 added section on deprecated constructor

Road-toward-3.0.0.md

@@ -13,4 +13,38 @@ In case of bugs, there may be another 2.x.x release. However, pushing forward ve
 
 # Changes
 
-This section shall list the changes that have been made past version 2.1.1. If possible, we shall provide examples how to change existing tools to work with the 3.0.0 release.
+This section shall list the changes that have been made past version 2.1.1. If possible, we shall provide examples how to change existing tools to work with the 3.0.0 release.
+
+## Removal of deprecated input stream constructor
+
+[Issue 480](https://github.com/nlohmann/json/issues/480) describes the removal of constructor `explicit basic_json(std::istream& i, const parser_callback_t cb = nullptr)`.
+
+### Code changes
+
+New code should use the `parse()` function. Let `ss` be a stream and `cb` be a parse callback function.
+
+Old code:
+
+```cpp
+json j(ss, cb);
+```
+
+New code:
+
+```cpp
+json j = json::parse(ss, cb);
+```
+
+If no callback function is used, also the following code works:
+
+```cpp
+json j;
+j << ss;
+```
+
+or
+
+```cpp
+json j;
+ss >> j;
+```