Add a readme for the cpp api

Author: ogoffart

api/sixtyfps-cpp/README.md

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+# SixtyFPS-cpp
+
+**A C++ UI toolkit**
+
+SixtyFPS is a GUI engine, with libraries for different languages.
+SixtyFPS.cpp the a C++ API to interact with a SictyFPS UI from your C++ program.
+
+## Tutorial
+
+Let's make a UI for a todo list application using the SixtyFPS UI description language.
+Hopefully this should be self explainatory. Check out the documentation of the language for help
+
+NOTE: this is not yet implemented as is.
+
+```sixtyfps
+// file: todoapp.60
+TodoApp := MainWindow {
+    signal todo_added(string);
+    property<model> todo_model;
+
+    ColumnLayout {
+        RowLayout {
+            text_edit := LineEdit {}
+            Button {
+                text: "Add Todo";
+                clicked => {
+                    todo_added(text_edit.text);
+                    text_edit.text = "";
+                }
+            }
+        }
+        NativeListView {
+            model: todo_model;
+        }
+    }
+}
+```
+
+Now, we can generate the C++ code using the following command
+
+```
+sixtyfpscpp_compiler todoapp.sixtyfps -o todoapp.h
+```
+
+Note: You would usually not type this command yourself, this is done automatically by the build system
+See the documentation for how to integrate with cmake
+
+This will generate a todoapp.h header file. It basically contains the following code
+(edited for briefty)
+
+```C++
+#include <sixtyfps>
+struct TodoApp : sixtyfps::window {
+    sixtyfps::signal<std::string_view> &todo_added();
+    sixtyfps::property<std::shared_ptr<sixtyfps::data_model<
+        sixtyfps::native_list_view_item>>> &todo_model();
+    //...
+}
+```
+
+We can then use this from out .cpp file
+
+```C++
+// include the generated file
+#include "todoapp.h"
+
+int main() {
+    // Let's instantiate our window: this return a handle to it
+    auto todo_app = sixtyfps::create_window<TodoApp>();
+
+    // let's create a model: `simple_data_model` is a data model which is simply backed by
+    // a vector behind the scene.
+    auto model = std::make_shared<sixtyfps::simple_data_model<sixtyfps::native_list_view_item>>();
+    model->push_back({"Write documentation", sixtyfps::native_list_view_item::checkable });
+    todo_app->data_model().set(model);
+
+    // let's connect our "add" button to add an item in the model
+    todo_app->todo_added().connect([=](std::string_view data) {
+        model->push_back({data, sixtyfps::native_list_view_item::checkable})
+    });
+
+    // Show the window
+    todo_app->show();
+
+    // Run the sixtyfps envent loop on this thread.
+    sixtyfps::run();
+}
+```
+
+That's it.
+
+Check the rest of the documentation for the reference.