Add a "Breaking changes" section to the changelog

CHANGELOG.md

@@ -29,3 +29,83 @@ HEAD
 * `serializeJson()` and `serializeMsgPack()` replace the content of `std::string` and `String` instead of appending to it
 * Replace `add()` with `add<T>()` (`add(T)` is still supported)
 * Remove `createNestedArray()` and `createNestedObject()` (use `to<JsonArray>()` and `to<JsonObject>()` instead)
+
+> ### BREAKING CHANGES
+>
+> As every major release, ArduinoJson 7 introduces several breaking changes.
+> I added some stubs so that most existing programs should compile, but I highty recommend you upgrade your code.
+>
+> #### `JsonDocument`
+> 
+> In ArduinoJson 6, you could allocate the memory pool on the stack (with `StaticJsonDocument`) or in the heap (with `DynamicJsonDocument`).  
+> In ArduinoJson 7, the memory pool is always allocated in the heap, so `StaticJsonDocument` and `DynamicJsonDocument` have been merged into `JsonDocument`.
+>
+> In ArduinoJson 6, `JsonDocument` had a fixed capacity; in ArduinoJson 7, it has an elastic capacity that grows as needed.
+> Therefore, you don't need to specify the capacity anymore, so the macros `JSON_ARRAY_SIZE()`, `JSON_OBJECT_SIZE()`, and `JSON_STRING_SIZE()` have been removed.
+>
+> ```c++
+> // ArduinoJson 6
+> StaticJsonDocument<256> doc;
+> // or
+> DynamicJsonDocument doc(256);
+> 
+> // ArduinoJson 7
+> JsonDocument doc;
+> ```
+>
+> In ArduinoJson 7, `JsonDocument` reuses released memory, so `garbageCollect()` has been removed.  
+> `shrinkToFit()` is still available and releases the over-allocated memory.
+>
+> Due to a change in the implementation, it's not possible to store a pointer to a variant from another `JsonDocument`, so `shallowCopy()` has been removed.
+> 
+> In ArduinoJson 6, the meaning of `memoryUsage()` was clear: it returned the number of bytes used in the memory pool.  
+> In ArduinoJson 7, the meaning of `memoryUsage()` would be ambiguous, so it has been removed.
+>
+> #### Custom allocators
+>
+> In ArduinoJson 6, you could specify a custom allocator class as a template parameter of `BasicJsonDocument`.  
+> In ArduinoJson 7, you must inherit from `ArduinoJson::Allocator` and pass a pointer to an instance of your class to the constructor of `JsonDocument`.
+>
+> ```c++
+> // ArduinoJson 6
+> class MyAllocator {
+>   // ...
+> };
+> BasicJsonDocument<MyAllocator> doc(256);
+>
+> // ArduinoJson 7
+> class MyAllocator : public ArduinoJson::Allocator {
+>   // ...
+> };
+> MyAllocator myAllocator;
+> JsonDocument doc(&myAllocator);
+> ```
+>
+> #### `createNestedArray()` and `createNestedObject()`
+>
+> In ArduinoJson 6, you could create a nested array or object with `createNestedArray()` and `createNestedObject()`.  
+> In ArduinoJson 7, you must use `add<T>()` or `to<T>()` instead.
+>
+> For example, to create `[[],{}]`, you would write:
+>
+> ```c++
+> // ArduinoJson 6
+> doc.createNestedArray("array");
+> doc.createNestedObject("object");
+>
+> // ArduinoJson 7
+> doc.add<JsonArray>();
+> doc.add<JsonObject>();
+> ```
+>
+> And to create `{"array":[],"object":{}}`, you would write:
+>
+> ```c++
+> // ArduinoJson 6
+> doc.createNestedArray("array");
+> doc.createNestedObject("object");
+>
+> // ArduinoJson 7
+> doc["array"].to<JsonArray>();
+> doc["object"].to<JsonObject>();
+> ```