add grass entity preview in readme
add grass entity preview in readme

jansedivy/opengl-game,jansedivy/opengl-game,jansedivy/opengl-game,jansedivy/opengl-game,jansedivy/opengl-game,jansedivy/opengl-game,jansedivy/opengl-game,jansedivy/opengl-game,jansedivy/opengl-game