MINOR: Update the README.md to include a note about GRADLE_USER_HOME
MINOR: Update the README.md to include a note about GRADLE_USER_HOME

Trying to build the source and publish it to internal Maven repo, I ran into an issue that I explain in the mailing list discussion here https://www.mail-archive.com/devkafka.apache.org/msg56359.html.

The commit here updates the README.md to make a note that the GRADLE_USER_HOME environment variable plays a role in deciding which file to add the maven configs to.

Author: Jaikiran Pai <jaikiran.pai@gmail.com>

Reviewers: Ismael Juma <ismael@juma.me.uk>

Closes #1837 from jaikiran/readme-update-grade-user-home

ijuma/kafka,KevinLiLu/kafka,geeag/kafka,noslowerdna/kafka,guozhangwang/kafka,sslavic/kafka,ErikKringen/kafka,sebadiaz/kafka,Ishiihara/kafka,Chasego/kafka,gf53520/kafka,richhaase/kafka,noslowerdna/kafka,sebadiaz/kafka,ErikKringen/kafka,rhauch/kafka,ErikKringen/kafka,sebadiaz/kafka,gf53520/kafka,Chasego/kafka,rhauch/kafka,rhauch/kafka,KevinLiLu/kafka,Chasego/kafka,geeag/kafka,apache/kafka,sslavic/kafka,lindong28/kafka,Esquive/kafka,eribeiro/kafka,themarkypantz/kafka,geeag/kafka,MyPureCloud/kafka,noslowerdna/kafka,noslowerdna/kafka,TiVo/kafka,TiVo/kafka,apache/kafka,lindong28/kafka,guozhangwang/kafka,mihbor/kafka,mihbor/kafka,eribeiro/kafka,richhaase/kafka,Ishiihara/kafka,airbnb/kafka,gf53520/kafka,ollie314/kafka,MyPureCloud/kafka,Esquive/kafka,geeag/kafka,lindong28/kafka,ijuma/kafka,KevinLiLu/kafka,apache/kafka,themarkypantz/kafka,lindong28/kafka,MyPureCloud/kafka,ijuma/kafka,guozhangwang/kafka,gf53520/kafka,KevinLiLu/kafka,apache/kafka,mihbor/kafka,themarkypantz/kafka,ollie314/kafka,Ishiihara/kafka,Chasego/kafka,guozhangwang/kafka,ijuma/kafka,Esquive/kafka,Ishiihara/kafka,themarkypantz/kafka,ErikKringen/kafka,sebadiaz/kafka,richhaase/kafka,airbnb/kafka,sslavic/kafka,TiVo/kafka,Esquive/kafka,eribeiro/kafka,ollie314/kafka,richhaase/kafka,airbnb/kafka,sslavic/kafka,TiVo/kafka,MyPureCloud/kafka,rhauch/kafka,mihbor/kafka,airbnb/kafka,eribeiro/kafka,ollie314/kafka