MINOR: Update README to specify Gradle 4.6 as the minimum required version (#5606)
MINOR: Update README to specify Gradle 4.6 as the minimum required version (#5606)

#5602 uses `annotationProcessor`, which was introduced in Gradle 4.6.

Reviewers: Ismael Juma <0794a5c7e497910d682bf2c54c66cc30760c0221@juma.me.uk>
noslowerdna/kafka,Chasego/kafka,sslavic/kafka,lindong28/kafka,Chasego/kafka,TiVo/kafka,TiVo/kafka,sslavic/kafka,lindong28/kafka,gf53520/kafka,KevinLiLu/kafka,noslowerdna/kafka,Chasego/kafka,gf53520/kafka,mihbor/kafka,lindong28/kafka,mihbor/kafka,noslowerdna/kafka,guozhangwang/kafka,mihbor/kafka,KevinLiLu/kafka,lindong28/kafka,gf53520/kafka,apache/kafka,TiVo/kafka,TiVo/kafka,guozhangwang/kafka,sslavic/kafka,KevinLiLu/kafka,apache/kafka,mihbor/kafka,noslowerdna/kafka,Chasego/kafka,apache/kafka,KevinLiLu/kafka,guozhangwang/kafka,sslavic/kafka,gf53520/kafka,guozhangwang/kafka,apache/kafka