Added optional object based API for C++.

Change-Id: If927f3ea3fb3723088fa287f24bdd1ad43c8d1d1
Tested: on Linux.

docs/source/CppUsage.md

@@ -85,6 +85,27 @@ convenient accessors for all fields, e.g. `hp()`, `mana()`, etc:
 
 *Note: That we never stored a `mana` value, so it will return the default.*
 
+## Object based API.
+
+FlatBuffers is all about memory efficiency, which is why its base API is written
+around using as little as possible of it. This does make the API clumsier
+(requiring pre-order construction of all data, and making mutation harder).
+
+For times when efficiency is less important a more convenient object based API
+can be used (through `--gen-object-api`) that is able to unpack & pack a
+FlatBuffer into objects and standard STL containers, allowing for convenient
+construction, access and mutation.
+
+To use:
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
+    auto monsterobj = GetMonster(buffer)->UnPack();
+    cout << monsterobj->name;  // This is now a std::string!
+    monsterobj->name = "Bob";  // Change the name.
+    FlatBufferBuilder fbb;
+    monsterobj->Pack(fbb);     // Serialize into new buffer.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
 ## Reflection (& Resizing)
 
 There is experimental support for reflection in FlatBuffers, allowing you to