Add docs for graphics effects

Author: adazem009

docs/Graphics effects.md

@@ -0,0 +1,38 @@
+\page graphicsEffects Graphics effects
+
+libscratchcpp provides API for implementing custom graphics effects.
+No effects are currently included with libscratchcpp, but that is planned to change.
+
+In case you've implemented some of the effects yourself, feel free to make pull requests!
+These are the issues (tasks) for the default graphics effects:
+
+- Color: https://github.com/scratchcpp/libscratchcpp/issues/283
+- Fisheye: https://github.com/scratchcpp/libscratchcpp/issues/284
+- Whirl: https://github.com/scratchcpp/libscratchcpp/issues/285
+- Pixelate: https://github.com/scratchcpp/libscratchcpp/issues/286
+- Mosaic: https://github.com/scratchcpp/libscratchcpp/issues/287
+- Brightness: https://github.com/scratchcpp/libscratchcpp/issues/288
+- Ghost: https://github.com/scratchcpp/libscratchcpp/issues/289
+
+# Implementing a graphics effect
+To implement a graphics effect that libscratchcpp doesn't support,
+subclass \link libscratchcpp::IGraphicsEffect IGraphicsEffect \endlink
+and override all of the methods.
+
+It's recommended to use the libscratchcpp namespace like this in your class:
+```cpp
+using namespace libscratchcpp;
+```
+
+The `name()` method should return the name of the effect which is
+used by the set and change effect blocks, for example `ghost` or `fisheye`.
+
+The `apply()` method is used to apply the effect on a bitmap. The bitmap
+can be accessed using the `bitmap` parameter and is writable.
+
+# Registering the graphics effect
+To register the graphics effect, use \link libscratchcpp::ScratchConfiguration::registerGraphicsEffect() ScratchConfiguration::registerGraphicsEffect() \endlink.
+
+```cpp
+libscratchcpp::ScratchConfiguration::registerGraphicsEffect(std::make_shared<MyGraphicsEffect>());
+```