Mention in README global workspace management

Global workspace management is controlled by an environment variable
called POSTGRES_JENKINS_WORKSPACE. Installation binaries are located
in $POSTGRES_JENKINS_WORKSPACE/install. Data folder are located in
$POSTGRES_JENKINS_WORKSPACE/data. Default value should be set at
/var/lib/jenkins/global but this can be changed at will depending on
distribution and deployment spec. Just be sure that this variable is
loaded with /etc/profile or similar.