Documentation: extensions: add doc about native-object

Documentation/extensions/native-object.md

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+# GetNativeObject API
+
+To be able to use platform-specific features or get optional engine interfaces, we've added a simple call to MobilityAPI on client DLL and PhysicsAPI for server DLL and extended MenuAPI for menu DLL.
+
+It's defined like this:
+
+```
+void *pfnGetNativeObject( const char *name );
+```
+
+#### Cross-platform objects
+
+Only these objects are guaranteed to be available on all targets.
+
+| Object name | Interface |
+|-------------|-----------|
+| `VFileSystem009` | Provides C++ interface to filesystem, binary-compatible with Valve's VFileSystem009. |
+| `XashFileSystemXXX` | Provides C interface to filesystem. This interface is unstable and not recommended for generic use, outside of engine internals. For more info about current version look into `filesystem.h`. |
+
+#### Android-specific objects
+
+| Object name | Interface |
+|-------------|-----------|
+| `JNIEnv`    | Allows interfacing with Java Native Interface. |
+| `ActivityClass` | Returns JNI object for engine Android activity class. |