Added command in readme to build playcanvas-extras (#2099)
Added command in readme to build playcanvas-extras (#2099)

* Added command to build playcanvas-extras

* Update README.md

Co-authored-by: Steven Yau <c4e01c0f87a9d4bf034f0f681553ace272d52b0f@snaphcat.com>
Co-authored-by: Will Eastcott <37d41699bdee4fcb969ca499eb0f8b82c60d59cc@playcanvas.com>
sereepap2029/playcanvas,sereepap2029/playcanvas,playcanvas/engine,aidinabedi/playcanvas-engine,playcanvas/engine,MicroWorldwide/PlayCanvas,sereepap2029/playcanvas,aidinabedi/playcanvas-engine,MicroWorldwide/PlayCanvas