Update document and bump version to 1.5.20
Update document and bump version to 1.5.20

aknuds1/s4cmd,deepsentinel/s4cmd,npyoung/s4cmd,sysalexis/s4cmd,bloomreach/s4cmd,deepsentinel/s4cmd,jephdo/s4cmd,Nmishin/s4cmd,Nmishin/s4cmd,npyoung/s4cmd,jephdo/s4cmd,bloomreach/s4cmd,aknuds1/s4cmd,sysalexis/s4cmd