Doc: Document list of supported environmental varaibles. (#2864)
Doc: Document list of supported environmental varaibles. (#2864)

fixes #2773
donatello/minio,alexellis/minio,fwessels/minio,harshavardhana/minio,fwessels/minio,brendanashworth/minio,dvstate/minio,rushenn/minio,dutchcoders/minio,dutchcoders/minio,donatello/minio,krishnasrinivas/minio-daily,luomeiqin/minio,minio/minio,rushenn/minio,harshavardhana/minio,luomeiqin/minio,aead/minio,mf-00/newgo,krishnasrinivas/minio-daily,rushenn/minio,harshavardhana/minio,krishnasrinivas/minio,aead/minio,brendanashworth/minio,fwessels/minio,hackintoshrao/minio,dutchcoders/minio,mf-00/newgo,NitishT/minio,krisis/minio,dvstate/minio,dutchcoders/minio,krishnasrinivas/minio,krisis/minio,krishnasrinivas/minio,aead/minio,aead/minio,harshavardhana/minio,NitishT/minio,koolhead17/minio,balamurugana/minio,luomeiqin/minio,fwessels/minio,harshavardhana/minio,hackintoshrao/minio,jimjag/minio,aead/minio,minio/minio,NitishT/minio,harshavardhana/minio,abperiasamy/minio,brendanashworth/minio,luomeiqin/minio,jimjag/minio,abperiasamy/minio,rushenn/minio,harshavardhana/minio,balamurugana/minio,dvstate/minio,vadmeste/minio,aead/minio,aead/minio,jimjag/minio,alexellis/minio,dvstate/minio,hackintoshrao/minio,abperiasamy/minio,harshavardhana/minio,jimjag/minio,NitishT/minio,brendanashworth/minio,krishnasrinivas/minio,abperiasamy/minio,hackintoshrao/minio,aead/minio,koolhead17/minio,vadmeste/minio