Update documentation on usage

[leavittx]

Hello!
Wouldn't it be cool to add the steps required in order to get the whole transparent window thing working to the repository README.md?
The 1st step is to uncheck the "Use DXGI Flip Model Swapchain for D3D11" checkbox in the player settings.
The 2nd step is to set the main camera Clear Flags to Solid Color and the Background to 00000000 in hex.
After that it's trivial to get it working.