Updated README to add optional parameters

Author: rpapallas

README.md

@@ -48,3 +48,9 @@ Double-click on a geom and hold `Ctrl` to apply forces (right) and torques (left
 
 Press `ESC` to quit.  
 Other key bindings are shown in the overlay menu (almost similar to `mujoco-py`).
+
+# Optional Parameters
+
+- `title`: set the title of the window, for example: `viewer = mujoco_viewer.MujocoViewer(model, data, title='My Demo')` (defaults to `mujoco-python-viewer`). 
+- `width`: set the window width, for example: `viewer = mujoco_viewer.MujocoViewer(model, data, width=300)` (defaults to full screen's width). 
+- `height`: set the window height, for example: `viewer = mujoco_viewer.MujocoViewer(model, data, height=300)` (defaults to full screen's height).