Add a reference to Predis\Async in the README.
Add a reference to Predis\Async in the README.

I think we can indeed start promoting Predis\Async now that it has
reached a fairly stable state (in terms of API and functionalites)
despite still being considered experimental.

[ci skip]

nguyenthaihan/predis,protomouse/predis,mrkeng/predis,gopalindians/predis,RudyJessop/predis,CloudSide/predis,SecureCloud-biz/predis,gencer/predis,zhangyancoder/predis,vend/predis,WalterShe/predis,stokes3452/predis,oswaldderiemaecker/predis,moria/predis,RedisLabs/predis,dzung2t/predis,channelgrabber/predis,dominics/predis,SwelenFrance/predis,nrk/predis,johnhelmuth/predis,lvbaosong/predis,quangnguyen90/predis