update v4.1.0

add fps property

Author: drawcall

README.md

@@ -106,6 +106,8 @@ proton.addRenderer(renderer);
 
 * Added `Proton.Cyclone` behavior, you can make vortex effects with Cyclone. Demo please check [here](https://codesandbox.io/s/proton-cyclone-rzweu).
 
+* `proton.fps=60` In most cases, you don't need to set this property. You can set this property when the game engine has fixed fps or some browsers have a higher refresh rate.
+
 * Use Euler integration calculation is more accurate (default false) `Proton.USE_CLOCK = false or true;`.
 
 Proton has now been upgraded to the __v4__ version. Performance has been greatly improved and api also has some improvements. For more details, please check [here](https://github.com/a-jie/Proton/releases).