Update README.md

Author: jmitrevs

README.md

@@ -2,3 +2,13 @@
 A QtQuick backend for matplotlib
 
 Started from maplotlib_qtquick_playground on Frédéric Collonval, but updated to work with the current version (3.2.1) of matplotlib.
+
+## Usage
+
+In order to use matplotlib plots with QtQuick, do the following:
+
+1. In your main.py, use `QtQml.qmlRegisterType` to register `FigureCanvasQtQuickAgg` under whichever name you want to use.
+1. In your QML files, instantiate an object with the QML type defined above. Its `objectName` property specifies the name of the object that can be found in the QML engine. It can be retrieved in python with `win.findChild(QtCore.QObject, "spectFigure")`, for example, where `"spectFigure"` is the `objectName` in this case.
+1. One can then pass this object as needed. This is the FigureCanvasQtQuickAgg object. The figure can be found in its `figure` property
+
+Please let me know if you have any suggestions or better ways I should do this.